Code Path Analysis Details

ESLint’s rules can use code paths. The code path is execution routes of programs. It forks/joins at such as if statements.

if (a && b) {
    foo();
}
bar();
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Objects

Program is expressed with several code paths. A code path is expressed with objects of two kinds: CodePath and CodePathSegment.

CodePath

CodePath expresses whole of one code path. This object exists for each function and the global. This has references of both the initial segment and the final segments of a code path.

CodePath has the following properties:

• id (string) - A unique string. Respective rules can use id to save additional information for each code path.
• origin (string) - The reason that the code path was started. May be "program", "function", "class-field-initializer", or "class-static-block".
• initialSegment (CodePathSegment) - The initial segment of this code path.
• finalSegments (CodePathSegment[]) - The final segments which includes both returned and thrown.
• returnedSegments (CodePathSegment[]) - The final segments which includes only returned.
• thrownSegments (CodePathSegment[]) - The final segments which includes only thrown.
• upper (CodePath|null) - The code path of the upper function/global scope.
• childCodePaths (CodePath[]) - Code paths of functions this code path contains.

CodePathSegment

CodePathSegment is a part of a code path. A code path is expressed with plural CodePathSegment objects, it’s similar to doubly linked list. Difference from doubly linked list is what there are forking and merging (the next/prev are plural).

CodePathSegment has the following properties:

• id (string) - A unique string. Respective rules can use id to save additional information for each segment.
• nextSegments (CodePathSegment[]) - The next segments. If forking, there are two or more. If final, there is nothing.
• prevSegments (CodePathSegment[]) - The previous segments. If merging, there are two or more. If initial, there is nothing.
• reachable (boolean) - A flag which shows whether or not it’s reachable. This becomes false when preceded by return, throw, break, or continue.

Events

There are seven events related to code paths, and you can define event handlers by adding them alongside node visitors in the object exported from the create() method of your rule.

module.exports = {
    meta: {
        // ...
    },
    create(context) {

        return {
            /**
             * This is called at the start of analyzing a code path.
             * In this time, the code path object has only the initial segment.
             *
             * @param {CodePath} codePath - The new code path.
             * @param {ASTNode} node - The current node.
             * @returns {void}
             */
            onCodePathStart(codePath, node) {
                // do something with codePath
            },

            /**
             * This is called at the end of analyzing a code path.
             * In this time, the code path object is complete.
             *
             * @param {CodePath} codePath - The completed code path.
             * @param {ASTNode} node - The current node.
             * @returns {void}
             */
            onCodePathEnd(codePath, node) {
                // do something with codePath
            },

            /**
             * This is called when a reachable code path segment was created.
             * It meant the code path is forked or merged.
             * In this time, the segment has the previous segments and has been
             * judged reachable or not.
             *
             * @param {CodePathSegment} segment - The new code path segment.
             * @param {ASTNode} node - The current node.
             * @returns {void}
             */
            onCodePathSegmentStart(segment, node) {
                // do something with segment
            },

            /**
             * This is called when a reachable code path segment was left.
             * In this time, the segment does not have the next segments yet.
             *
             * @param {CodePathSegment} segment - The left code path segment.
             * @param {ASTNode} node - The current node.
             * @returns {void}
             */
            onCodePathSegmentEnd(segment, node) {
                // do something with segment
            },

            /**
             * This is called when an unreachable code path segment was created.
             * It meant the code path is forked or merged.
             * In this time, the segment has the previous segments and has been
             * judged reachable or not.
             *
             * @param {CodePathSegment} segment - The new code path segment.
             * @param {ASTNode} node - The current node.
             * @returns {void}
             */
            onUnreachableCodePathSegmentStart(segment, node) {
                // do something with segment
            },

            /**
             * This is called when an unreachable code path segment was left.
             * In this time, the segment does not have the next segments yet.
             *
             * @param {CodePathSegment} segment - The left code path segment.
             * @param {ASTNode} node - The current node.
             * @returns {void}
             */
            onUnreachableCodePathSegmentEnd(segment, node) {
                // do something with segment
            },

            /**
             * This is called when a code path segment was looped.
             * Usually segments have each previous segments when created,
             * but when looped, a segment is added as a new previous segment into a
             * existing segment.
             *
             * @param {CodePathSegment} fromSegment - A code path segment of source.
             * @param {CodePathSegment} toSegment - A code path segment of destination.
             * @param {ASTNode} node - The current node.
             * @returns {void}
             */
            onCodePathSegmentLoop(fromSegment, toSegment, node) {
                // do something with segment
            }
        };

    }
}
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About onCodePathSegmentLoop

This event is always fired when the next segment has existed already. That timing is the end of loops mainly.

For Example 1:

while (a) {
    a = foo();
}
bar();
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1. First, the analysis advances to the end of loop.

2. Second, it creates the looping path. At this time, the next segment has existed already, so the onCodePathSegmentStart event is not fired. It fires onCodePathSegmentLoop instead.

3. Last, it advances to the end.

For example 2:

for (let i = 0; i < 10; ++i) {
    foo(i);
}
bar();
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1. for statements are more complex. First, the analysis advances to ForStatement.update. The update segment is hovered at first.

2. Second, it advances to ForStatement.body. Of course the body segment is preceded by the test segment. It keeps the update segment hovering.

3. Third, it creates the looping path from body segment to update segment. At this time, the next segment has existed already, so the onCodePathSegmentStart event is not fired. It fires onCodePathSegmentLoop instead.

4. Fourth, also it creates the looping path from update segment to test segment. At this time, the next segment has existed already, so the onCodePathSegmentStart event is not fired. It fires onCodePathSegmentLoop instead.

5. Last, it advances to the end.

Usage Examples

Track current segment position

To track the current code path segment position, you can define a rule like this:

module.exports = {
    meta: {
        // ...
    },
    create(context) {

        // tracks the code path we are currently in
        let currentCodePath;

        // tracks the segments we've traversed in the current code path
        let currentSegments;

        // tracks all current segments for all open paths
        const allCurrentSegments = [];

        return {

            onCodePathStart(codePath) {
                currentCodePath = codePath;
                allCurrentSegments.push(currentSegments);
                currentSegments = new Set();
            },

            onCodePathEnd(codePath) {
                currentCodePath = codePath.upper;
                currentSegments = allCurrentSegments.pop();
            },

            onCodePathSegmentStart(segment) {
                currentSegments.add(segment);
            },

            onCodePathSegmentEnd(segment) {
                currentSegments.delete(segment);
            },

            onUnreachableCodePathSegmentStart(segment) {
                currentSegments.add(segment);
            },

            onUnreachableCodePathSegmentEnd(segment) {
                currentSegments.delete(segment);
            }
        };

    }
};
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In this example, the currentCodePath variable is used to access the code path that is currently being traversed and the currentSegments variable tracks the segments in that code path that have been traversed to that point. Note that currentSegments both starts and ends as an empty set, constantly being updated as the traversal progresses.

Tracking the current segment position is helpful for analyzing the code path that led to a particular node, as in the next example.

Find an unreachable node

To find an unreachable node, track the current segment position and then use a node visitor to check if any of the segments are reachable. For example, the following looks for any ExpressionStatement that is unreachable.

function areAnySegmentsReachable(segments) {
    for (const segment of segments) {
        if (segment.reachable) {
            return true;
        }
    }

    return false;
}

module.exports = {
    meta: {
        // ...
    },
    create(context) {

        // tracks the code path we are currently in
        let currentCodePath;

        // tracks the segments we've traversed in the current code path
        let currentSegments;

        // tracks all current segments for all open paths
        const allCurrentSegments = [];

        return {

            onCodePathStart(codePath) {
                currentCodePath = codePath;
                allCurrentSegments.push(currentSegments);
                currentSegments = new Set();
            },

            onCodePathEnd(codePath) {
                currentCodePath = codePath.upper;
                currentSegments = allCurrentSegments.pop();
            },

            onCodePathSegmentStart(segment) {
                currentSegments.add(segment);
            },

            onCodePathSegmentEnd(segment) {
                currentSegments.delete(segment);
            },

            onUnreachableCodePathSegmentStart(segment) {
                currentSegments.add(segment);
            },

            onUnreachableCodePathSegmentEnd(segment) {
                currentSegments.delete(segment);
            },

            ExpressionStatement(node) {

                // check all the code path segments that led to this node
                if (!areAnySegmentsReachable(currentSegments)) {
                    context.report({ message: "Unreachable!", node });
                }
            }

        };

    }
};
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See Also: no-unreachable, no-fallthrough, consistent-return

Check if a function is called in every path

This example checks whether or not the parameter cb is called in every path. Instances of CodePath and CodePathSegment are shared to every rule. So a rule must not modify those instances. Please use a map of information instead.

function hasCb(node, context) {
    if (node.type.indexOf("Function") !== -1) {
        const sourceCode = context.sourceCode;
        return sourceCode.getDeclaredVariables(node).some(function(v) {
            return v.type === "Parameter" && v.name === "cb";
        });
    }
    return false;
}

function isCbCalled(info) {
    return info.cbCalled;
}

module.exports = {
    meta: {
        // ...
    },
    create(context) {

        let funcInfo;
        const funcInfoStack = [];
        const segmentInfoMap = Object.create(null);

        return {
            // Checks `cb`.
            onCodePathStart(codePath, node) {
                funcInfoStack.push(funcInfo);

                funcInfo = {
                    codePath: codePath,
                    hasCb: hasCb(node, context),
                    currentSegments: new Set()
                };
            },

            onCodePathEnd(codePath, node) {
                funcInfo = funcInfoStack.pop();

                // Checks `cb` was called in every paths.
                const cbCalled = codePath.finalSegments.every(function(segment) {
                    const info = segmentInfoMap[segment.id];
                    return info.cbCalled;
                });

                if (!cbCalled) {
                    context.report({
                        message: "`cb` should be called in every path.",
                        node: node
                    });
                }
            },

            // Manages state of code paths and tracks traversed segments
            onCodePathSegmentStart(segment) {

                funcInfo.currentSegments.add(segment);

                // Ignores if `cb` doesn't exist.
                if (!funcInfo.hasCb) {
                    return;
                }

                // Initialize state of this path.
                const info = segmentInfoMap[segment.id] = {
                    cbCalled: false
                };

                // If there are the previous paths, merges state.
                // Checks `cb` was called in every previous path.
                if (segment.prevSegments.length > 0) {
                    info.cbCalled = segment.prevSegments.every(isCbCalled);
                }
            },

            // Tracks unreachable segment traversal
            onUnreachableCodePathSegmentStart(segment) {
                funcInfo.currentSegments.add(segment);
            },

            // Tracks reachable segment traversal
            onCodePathSegmentEnd(segment) {
                funcInfo.currentSegments.delete(segment);
            },

            // Tracks unreachable segment traversal
            onUnreachableCodePathSegmentEnd(segment) {
                funcInfo.currentSegments.delete(segment);
            },

            // Checks reachable or not.
            CallExpression(node) {

                // Ignores if `cb` doesn't exist.
                if (!funcInfo.hasCb) {
                    return;
                }

                // Sets marks that `cb` was called.
                const callee = node.callee;
                if (callee.type === "Identifier" && callee.name === "cb") {
                    funcInfo.currentSegments.forEach(segment => {
                        const info = segmentInfoMap[segment.id];
                        info.cbCalled = true;
                    });
                }
            }
        };
    }
};
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See Also: constructor-super, no-this-before-super

Code Path Examples

Hello World

console.log("Hello world!");
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IfStatement

if (a) {
    foo();
} else {
    bar();
}
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IfStatement (chain)

if (a) {
    foo();
} else if (b) {
    bar();
} else if (c) {
    hoge();
}
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SwitchStatement

switch (a) {
    case 0:
        foo();
        break;

    case 1:
    case 2:
        bar();
        // fallthrough

    case 3:
        hoge();
        break;
}
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SwitchStatement (has default)

switch (a) {
    case 0:
        foo();
        break;

    case 1:
    case 2:
        bar();
        // fallthrough

    case 3:
        hoge();
        break;

    default:
        fuga();
        break;
}
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TryStatement (try-catch)

try {
    foo();
    if (a) {
        throw new Error();
    }
    bar();
} catch (err) {
    hoge(err);
}
last();
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It creates the paths from try block to catch block at:

• throw statements.
• The first throwable node (e.g. a function call) in the try block.
• The end of the try block.

TryStatement (try-finally)

try {
    foo();
    bar();
} finally {
    fuga();
}
last();
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If there is not catch block, finally block has two current segments. At this time when running the previous example to find unreachable nodes, currentSegments.length is 2. One is the normal path, and another is the leaving path (throw or return).

TryStatement (try-catch-finally)

try {
    foo();
    bar();
} catch (err) {
    hoge(err);
} finally {
    fuga();
}
last();
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WhileStatement

while (a) {
    foo();
    if (b) {
        continue;
    }
    bar();
}
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DoWhileStatement

do {
    foo();
    bar();
} while (a);
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ForStatement

for (let i = 0; i < 10; ++i) {
    foo();
    if (b) {
        break;
    }
    bar();
}
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ForStatement (for ever)

for (;;) {
    foo();
}
bar();
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ForInStatement

for (let key in obj) {
    foo(key);
}
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When there is a function

function foo(a) {
    if (a) {
        return;
    }
    bar();
}

foo(false);
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It creates two code paths.

• The global’s

• The function’s