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no-constant-condition

Disallow constant expressions in conditions

✅

Using the recommended config from @eslint/js in a configuration file enables this rule

A constant expression (for example, a literal) as a test condition might be a typo or development trigger for a specific behavior. For example, the following code looks as if it is not ready for production.

if (false) {
    doSomethingUnfinished();
}

Rule Details

This rule disallows constant expressions in the test condition of:

  • if, for, while, or do...while statement
  • ?: ternary expression

Examples of incorrect code for this rule:

Open in Playground 
/*eslint no-constant-condition: "error"*/

if (false) {
    doSomethingUnfinished();
}

if (void x) {
    doSomethingUnfinished();
}

if (x &&= false) {
    doSomethingNever();
}

if (class {}) {
    doSomethingAlways();
}

if (new Boolean(x)) {
    doSomethingAlways();
}

if (Boolean(1)) {
    doSomethingAlways();
}

if (undefined) {
    doSomethingUnfinished();
}

if (x ||= true) {
    doSomethingAlways();
}

for (;-2;) {
    doSomethingForever();
}

while (typeof x) {
    doSomethingForever();
}

do {
    doSomethingForever();
} while (x = -1);

const result = 0 ? a : b;

if(input === "hello" || "bye"){
  output(input);
}

Examples of correct code for this rule:

Open in Playground 
/*eslint no-constant-condition: "error"*/

if (x === 0) {
    doSomething();
}

for (;;) {
    doSomethingForever();
}

while (typeof x === "undefined") {
    doSomething();
}

do {
    doSomething();
} while (x);

const result = x !== 0 ? a : b;

if(input === "hello" || input === "bye"){
  output(input);
}

Options

checkLoops

This is a string option having following values:

  • "all" - Disallow constant expressions in all loops.
  • "allExceptWhileTrue" (default) - Disallow constant expressions in all loops except while loops with expression true.
  • "none" - Allow constant expressions in loops.

Or instead you can set the checkLoops value to booleans where true is same as "all" and false is same as "none".

Examples of incorrect code for when checkLoops is "all" or true:

Open in Playground 
/*eslint no-constant-condition: ["error", { "checkLoops": "all" }]*/

while (true) {
    doSomething();
};

for (;true;) {
    doSomething();
};
Open in Playground 
/*eslint no-constant-condition: ["error", { "checkLoops": true }]*/

while (true) {
    doSomething();
};

do {
    doSomething();
} while (true)

Examples of correct code for when checkLoops is "all" or true:

Open in Playground 
/*eslint no-constant-condition: ["error", { "checkLoops": "all" }]*/

while (a === b) {
    doSomething();
};
Open in Playground 
/*eslint no-constant-condition: ["error", { "checkLoops": true }]*/

for (let x = 0; x <= 10; x++) {
    doSomething();
};

Example of correct code for when checkLoops is "allExceptWhileTrue":

Open in Playground 
/*eslint no-constant-condition: "error"*/

while (true) {
    doSomething();
};

Examples of correct code for when checkLoops is "none" or false:

Open in Playground 
/*eslint no-constant-condition: ["error", { "checkLoops": "none" }]*/

while (true) {
    doSomething();
    if (condition()) {
        break;
    }
};

do {
    doSomething();
    if (condition()) {
        break;
    }
} while (true)
Open in Playground 
/*eslint no-constant-condition: ["error", { "checkLoops": false }]*/

while (true) {
    doSomething();
    if (condition()) {
        break;
    }
};

for (;true;) {
    doSomething();
    if (condition()) {
        break;
    }
};

Version

This rule was introduced in ESLint v0.4.1.

Resources

© OpenJS Foundation and other contributors
Licensed under the MIT License.
https://eslint.org/docs/latest/rules/no-constant-condition