Disallow unnecessary .call() and .apply(). (no-useless-call)

The function invocation can be written by Function.prototype.call() and Function.prototype.apply(). But Function.prototype.call() and Function.prototype.apply() are slower than the normal function invocation.

Rule Details

This rule is aimed to flag usage of Function.prototype.call() and Function.prototype.apply() that can be replaced with the normal function invocation.

The following patterns are considered warnings:

// Those are same as `foo(1, 2, 3);`
foo.call(undefined, 1, 2, 3);
foo.apply(undefined, [1, 2, 3]);
foo.call(null, 1, 2, 3);
foo.apply(null, [1, 2, 3]);
// Those are same as `obj.foo(1, 2, 3);`
obj.foo.call(obj, 1, 2, 3);
obj.foo.apply(obj, [1, 2, 3]);

The following patterns are not considered warnings:

// The `this` binding is different.
foo.call(obj, 1, 2, 3);
foo.apply(obj, [1, 2, 3]);
obj.foo.call(null, 1, 2, 3);
obj.foo.apply(null, [1, 2, 3]);
obj.foo.call(otherObj, 1, 2, 3);
obj.foo.apply(otherObj, [1, 2, 3]);
// The argument list is variadic.
foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);

Known limitations:

This rule compares code statically to check whether or not thisArg is changed. So if the code about thisArg is a dynamic expression, this rule cannot judge correctly.

// This is warned.
a[i++].foo.call(a[i++], 1, 2, 3);

// This is not warned.
a[++i].foo.call(a[i], 1, 2, 3);

When Not to Use It

If you don't want to be notified about unnecessary .call() and .apply(), you can safely disable this rule.

Version

This rule was introduced in ESLint 1.0.0-rc-1.

Resources