prefer-spread

Require spread operators instead of .apply()

Before ES2015, one must use Function.prototype.apply() to call variadic functions.

const args = [1, 2, 3, 4];
Math.max.apply(Math, args);
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In ES2015, one can use spread syntax to call variadic functions.

const args = [1, 2, 3, 4];
Math.max(...args);
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Rule Details

This rule is aimed to flag usage of Function.prototype.apply() in situations where spread syntax could be used instead.

Examples of incorrect code for this rule:

/*eslint prefer-spread: "error"*/

foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);
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Examples of correct code for this rule:

/*eslint prefer-spread: "error"*/

// Using spread syntax
foo(...args);
obj.foo(...args);

// The `this` binding is different.
foo.apply(obj, args);
obj.foo.apply(null, args);
obj.foo.apply(otherObj, args);

// The argument list is not variadic.
// Those are warned by the `no-useless-call` rule.
foo.apply(undefined, [1, 2, 3]);
foo.apply(null, [1, 2, 3]);
obj.foo.apply(obj, [1, 2, 3]);
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Options

This rule has no options.

Known Limitations

This rule analyzes code statically to check whether or not the this argument is changed. So, if the this argument is computed in a dynamic expression, this rule cannot detect a violation.

/*eslint prefer-spread: "error"*/

// This warns.
a[i++].foo.apply(a[i++], args);

// This does not warn.
a[++i].foo.apply(a[i], args);
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When Not To Use It

This rule should not be used in ES3/5 environments.

In ES2015 (ES6) or later, if you don’t want to be notified about Function.prototype.apply() callings, you can safely disable this rule.

Related Rules

• no-useless-call

Version

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

Resources

• Rule source
• Tests source