How does the following JavaScript syntax work? array['push']('5')

Question

While reverse-engineering some obfuscated JavaScript code for a CTF, I came across the following syntax:

array['push']('5');

It pushes '5' at the end of the array, which seems logical, but I just do not understand why this syntax works, since I haven't managed to find anything about it (not on the Mozilla Developer Network, not on the W3C website, and the latest ECMAScript specification is a bit too dense for me to understand).

I'm thinking it has something to do with arrays being a special case of objects, but I'm not versed enough in JavaScript to figure it out.

array['push'] means exactly the same thing as array.push. It has nothing to do with arrays being special. That's the normal and universal way that object property access works in JavaScript. — Pointy
– Pointy, Commented Oct 9, 2019 at 22:25

Barmar · Accepted Answer · 2019-10-09 22:33:51Z

3

In JavaScript, you can access object properties either using dot notation or square brackets. So object.propertyname is equivalent to object['propertyname']. Normally we use square brackets when the property name is calculated dynamically or is not a valid identifier (e.g. it contains special characters). But there's nothing prohibiting using it in other contexts. So array['push'] is equivalent to array.push, and therefore array['push'](5) is equivalent to array.push(5).

answered Oct 9, 2019 at 22:33

Barmar

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Geuis · Accepted Answer · 2019-10-10 21:39:32Z

0

array['push']('5'); translates to:

const array = [];
array.push('5');

Since push is always a method on arrays, it can be accessed via association like ['push'] or directly as a property.

edited Oct 10, 2019 at 21:39

answered Oct 9, 2019 at 22:33

Geuis

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3 Comments

Udochukwu Enwerem Over a year ago

The OP did not show where array was declared (as const). So, the translation isn't necessarily exact.

Jake Holzinger Over a year ago

5 is not '5'

Geuis Over a year ago

@UdoE. Its not supposed to be an exact translation. Its simply demonstrating the core concepts via code.

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