no-const-assign

Disallow reassigning const variables

We cannot modify variables that are declared using const keyword. It will raise a runtime error.

Under non ES2015 environment, it might be ignored merely.

Rule Details

This rule is aimed to flag modifying variables that are declared using const keyword.

Examples of incorrect code for this rule:

/*eslint no-const-assign: "error"*/
/*eslint-env es6*/

const a = 0;
a = 1;
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/*eslint no-const-assign: "error"*/
/*eslint-env es6*/

const a = 0;
a += 1;
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/*eslint no-const-assign: "error"*/
/*eslint-env es6*/

const a = 0;
++a;
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Examples of correct code for this rule:

/*eslint no-const-assign: "error"*/
/*eslint-env es6*/

const a = 0;
console.log(a);
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/*eslint no-const-assign: "error"*/
/*eslint-env es6*/

for (const a in [1, 2, 3]) { // `a` is re-defined (not modified) on each loop step.
    console.log(a);
}
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/*eslint no-const-assign: "error"*/
/*eslint-env es6*/

for (const a of [1, 2, 3]) { // `a` is re-defined (not modified) on each loop step.
    console.log(a);
}
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When Not To Use It

If you don’t want to be notified about modifying variables that are declared using const keyword, you can safely disable this rule.

Handled by TypeScript

It is safe to disable this rule when using TypeScript because TypeScript's compiler enforces this check.

Version

This rule was introduced in ESLint v1.0.0-rc-1.

Resources

• Rule source
• Tests source