Operator '==' cannot be applied to types string and number

Question

How to check the value of a number?

for(var index in this.choicesDM) {
  if(index == 0) {
    this.currentChoice = this.choicesDM[index]
  }
}

Here I get the error as mentioned in the title of the question. I also tried to explicitly define the type of index. But didn't work. Found this link but didn't got the solution.

Can you post the code which shows how you explicitly define the type of index? — Isaac, Apr 18 '18 at 02:57
Are you sure it is `index` and not a difference between `currentChoice and `choicesDM[index]`? — Isaac, Apr 18 '18 at 02:58

score 2 · Accepted Answer · answered Apr 18 '18 at 03:04

In JavaScript (and thus, TypeScript), the in operator returns a string. This is because for in loops iterate over an object's keys, which are strings in JavaScript/TypeScript. So, when you compare index == 0, index is a string and 0 is an integer, thus the error.

Note what I said before, though, that for in enumerates over an object's properties. In the case of arrays, this also includes things like the length property, so it's generally a bad idea to loop over arrays with for in. What you might want instead is a for of loop, where of gives you the value of each object in the array.

score 1 · Answer 2 · answered Apr 18 '18 at 02:57

1

The reason for this is because == and other related comparison operators operate on operands with same type or type of 1 operand should be subtype of other operand. But in your case 0 is integer and "index" is string. Hence the error.

answered Apr 18 '18 at 02:57

jat

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Operator '==' cannot be applied to types string and number

2 Answers2