什么样的JS对象是[foo：'bar']

Question

If I create a blank array and assign a value:

 let a = [] a['foo'] = 'bar' console.log(a) // [ foo: 'bar'] - not in SO console though console.log(a[0]) // undefined console.log(a.length) // 0 console.log(a instanceof Array) // true for (let i of a) {} // Doesn't loop but doesn't throw errors like {} for (let o in a) { console.log(o) // foo } 

What is this? An array?, of length 0? what kind of JS thing is this? also it will print to the node console but not recognized by the SO snippet console (run it and look at the web inspector console at same time)

Note that I don't need answers saying to initialize with {} . Thats not the point of the question.

Answer 1

Everything in javascript is an object so is the Array. Array is basically a zero-based keyed object that is iterable so when adding values with string keys into Array you're simply adding keys to that object.

In fact you can create your own object that is iterable just like Array and use it with for...of loops. Check this out: https://jsfiddle.net/gumh6bsy/

If you run let a = [ foo: 'two', bar: 'one' ] you get an error. So how does the loop in my code get that and print that to the console. Once the loop creates it it seems perfectly manageable. I thought at first it just transformed it into an object but it is an Array instance. – cyberwombat

let a = [ foo: 'two', bar: 'one' ] spits an error because square brackets are the syntax for initializing arrays and thus gets validated by javascript engine and does not allow definition of keys in the initialization. Meanwhile key assignment syntax ( a['foo'] = 'two' ) is not in anyway different for both arrays and objects. In fact it is just a syntax for assigning properties to an object. The main takeaway here is that in javascript array is an object of type Array . Actually defining array is pretty much the same as defining let a = { 0: 'one', 1: 'two' } except that Array (as a type) is recognized as "special" type in javascript that has special syntax for initialization ( let a = [ 'one', 'two', 'three' ] , gets zero-based numbered keys applied and it is iterable (can be used in for...of loops). Of course + all the prototype methods like find , filter , etc. Having this in mind adding string keyed property to an array is absolutely valid (not recommended) javascript and just because console.log applies special formatting when outputting arrays you get [ foo: 'one', bar: 'two' ] (with square brackets).

You can even whip-up some very rough implementation of array yourself, like: https://jsfiddle.net/t96ehm1r/ (of course without any syntax sugar for quick initialization and all the prototype methods, etc.)