Array.prototype[Symbol.unscopables]

A null-prototype object with property names given below and their values set to true.

The default Array properties that are ignored for with statement-binding purposes are:

• at()
• copyWithin()
• entries()
• fill()
• find()
• findIndex()
• findLast()
• findLastIndex()
• flat()
• flatMap()
• includes()
• keys()
• toReversed()
• toSorted()
• toSpliced()
• values()

Array.prototype[Symbol.unscopables] is an empty object only containing all the above property names with the value true. Its prototype is null, so Object.prototype properties like toString won't accidentally be made unscopable, and a toString() within the with statement will continue to be called on the array.

See Symbol.unscopables for how to set unscopable properties for your own objects.

Imagine the values.push('something') call below is in code that was written prior to ECMAScript 2015.

var values = [];

with (values) {
  values.push("something");
}

When ECMAScript 2015 introduced the Array.prototype.values() method, the with statement in the above code started to interpret values as the values.values array method instead of the external values variable. The values.push('something') call would break because it's now accessing push on the values.values method. This caused a bug to be reported to Firefox (Firefox Bug 883914).

So the [Symbol.unscopables] data property for Array.prototype causes the Array properties introduced in ECMAScript 2015 to be ignored for with statement-binding purposes — allowing code that was written prior to ECMAScript 2015 to continue working as expected, rather than breaking.

• Polyfill of Array.prototype[Symbol.unscopables] in core-js
• Indexed collections guide
• Array
• with
• Symbol.unscopables