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();

Code Path Example

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:

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:

Events

There are five events related to code paths, and you can define event handlers in rules.

module.exports = function(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": function(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": function(codePath, node) {
            // do something with codePath
        },

        /**
         * This is called when a 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": function(segment, node) {
            // do something with segment
        },

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

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();

  1. First, the analysis advances to the end of loop.

    Loop Event's Example 1

  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.

    Loop Event's Example 2

  3. Last, it advances to the end.

    Loop Event's Example 3

For example 2:

for (let i = 0; i < 10; ++i) {
    foo(i);
}
bar();

  1. for statements are more complex. First, the analysis advances to ForStatement.update. The update segment is hovered at first.

    Loop Event's Example 1

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

    Loop Event's Example 2

  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.

    Loop Event's Example 3

  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.

    Loop Event's Example 4

  5. Last, it advances to the end.

    Loop Event's Example 5

Usage Examples

To check whether or not this is reachable

var last = require("lodash").last;

function isReachable(segment) {
    return segment.reachable;
}

module.exports = function(context) {
    var codePathStack = [];

    return {
        // Stores CodePath objects.
        "onCodePathStart": function(codePath) {
            codePathStack.push(codePath);
        },
        "onCodePathEnd": function(codePath) {
            codePathStack.pop();
        },

        // Checks reachable or not.
        "ExpressionStatement": function(node) {
            var codePath = last(codePathStack);

            // Checks the current code path segments.
            if (!codePath.currentSegments.some(isReachable)) {
                context.report({message: "Unreachable!", node: node});
            }
        }
    };
};

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

To check state of a code path

This example is checking 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.

var last = require("lodash").last;

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

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

module.exports = function(context) {
    var funcInfoStack = [];
    var segmentInfoMap = Object.create(null);

    return {
        // Checks `cb`.
        "onCodePathStart": function(codePath, node) {
            funcInfoStack.push({
                codePath: codePath,
                hasCb: hasCb(node, context)
            });
        },
        "onCodePathEnd": function(codePath, node) {
            funcInfoStack.pop();

            // Checks `cb` was called in every paths.
            var cbCalled = codePath.finalSegments.every(function(segment) {
                var 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.
        "onCodePathSegmentStart": function(segment) {
            // Ignores if `cb` doesn't exist.
            if (!last(funcInfoStack).hasCb) {
                return;
            }

            // Initialize state of this path.
            var 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);
            }
        },

        // Checks reachable or not.
        "CallExpression": function(node) {
            var funcInfo = last(funcInfoStack);

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

            // Sets marks that `cb` was called.
            var callee = node.callee;
            if (callee.type === "Identifier" && callee.name === "cb") {
                funcInfo.codePath.currentSegments.forEach(function(segment) {
                    var info = segmentInfoMap[segment.id];
                    info.cbCalled = true;
                });
            }
        }
    };
};

See Also: constructor-super, no-this-before-super

Code Path Examples

Hello World

console.log("Hello world!");

Hello World

IfStatement

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

`IfStatement`

IfStatement (chain)

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

`IfStatement` (chain)

SwitchStatement

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

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

    case 3:
        hoge();
        break;
}

`SwitchStatement`

SwitchStatement (has default)

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

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

    case 3:
        hoge();
        break;

    default:
        fuga();
        break;
}

`SwitchStatement` (has `default`)

TryStatement (try-catch)

try {
    foo();
    if (a) {
        throw new Error();
    }
    bar();
} catch (err) {
    hoge(err);
}
last();

It creates the paths from try block to catch block at:

`TryStatement` (try-catch)

TryStatement (try-finally)

try {
    foo();
    bar();
} finally {
    fuga();
}
last();

If there is not catch block, finally block has two current segments. At this time, CodePath.currentSegments.length is 2. One is the normal path, and another is the leaving path (throw or return).

`TryStatement` (try-finally)

TryStatement (try-catch-finally)

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

`TryStatement` (try-catch-finally)

WhileStatement

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

`WhileStatement`

DoWhileStatement

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

`DoWhileStatement`

ForStatement

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

`ForStatement`

ForStatement (for ever)

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

`ForStatement` (for ever)

ForInStatement

for (let key in obj) {
    foo(key);
}

`ForInStatement`

When there is a function

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

foo(false);

It creates two code paths.