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Javascript – how to test whether a value is an array or object

I have an object that has several members, each an array. I have another object that includes this object. In the constructor for the outer object, I’d like to say that if the user passes in an object, I use that as the value for the inner object. If they pass in an array, I create a new object, using the passed-in array for one of the values and generating the other values by some rules for defaults.

That is:

class Outer {
  constructor(inner) {
    if (inner is an object) { // this is the statement I don't know how to write
      this.inner=inner
    }
    else {
      this.inner={"value1":inner}
      this.inner.value2=... blah blah blah
    }
  }

In my first draft I said, if (typeof inner == "object"). That didn’t work because for arrays it returns true. I guess an array is a special case of an object.

So I tried to do it the other way and say inner instanceof Array. That didn’t work because for objects it returned undefined.

So … is there some way that I can test if a value is an object versus an array?

If it’s relevant, in the new Outer I want to pass the value in as a literal, i.e. I might say new Outer([1,2,3]) or new Outer({"value1":[1,2,3],"value2":[2,3,4]).

And yes, I know I can solve the problem by just always passing in an object, and that’s what I’m going to do for at least the short term. I was thinking that the New Outer was cleaner if I could say new Outer([1,2,3]) rather than new Outer({"value1":[1,2,3]}). But it’s not that big a deal. But I’d like to know how to do this for future reference, at least.

Tags:

2

Answers


  1. 0

    Here is what you need to do.

    class Outer {
  constructor(inner) {
    if (Array.isArray(inner)) { 
      this.inner=inner
    }
    else {
      this.inner={"value1":inner}
      this.inner.value2=... blah blah blah
    }
  }

    Arrays are a special type of object only in JavaScript. So, when we use the typeof operator it gives object.

    console.log(type of someArray);
// Will give object

    But the Array class has some methods like isArray(), Which can be used to identify if something is an Array.

    Array.isArray(someArray)

    So, this function returns a boolean.

    console.log(Array.isArray(someArray))
// Will give true

    I hope it helps. Cheers 🤜🏻🤛🏻

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  2. 0

    You could check the prototype:

    class Outer {
  constructor(inner) {
    if (Object.getPrototypeOf(inner) === Object.prototype) {
      this.inner=inner
    }
    else {
      this.inner={"value1":inner}
    }
  }
}

console.log(new Outer({value1:[1,2,3]}));
console.log(new Outer([1,2,3]));

    Or use Array.isArray():

    class Outer {
  constructor(inner) {
    if (!Array.isArray(inner)) {
      this.inner=inner
    }
    else {
      this.inner={"value1":inner}
    }
  }
}

console.log(new Outer({value1:[1,2,3]}));
console.log(new Outer([1,2,3]));
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