Error in Javascript code

Question

I have a code snippet that searches a Json for the field called campo, and inserts it into an array, which cannot have repeated values. My array always returned Undefined and it took me a while to find the error.
Why do JS and even IDE consider this valid? If campo is an array, so it is obvious that after the . would call a method. In the browser there was no error.

Code snippet wrong:

while (i < listaContatos.length) {
   var dados = listaContatos[i].dados;
   for (var j = 0; j < dados.length; j++) {
       var nomeCampo = dados[j].campo;
       console.log(nomeCampo);
       if (campos.indexOf <= -1) {
           campos.push(nomeCampo);
         }
      }
   i++;
}

Answer 1

It is not considered an error for some reasons.

1. The point does not indicate that the method will be called, because for the call of a function the parentheses are mandatory. Otherwise you are just referencing the function as a variable. This makes it valid code because you are comparing a variable of type function with a variable of type number and Javascript does not restrict this due to its weakly typed nature. In this case, the comparison will always return false
2. Second, in the case of an array the point notation can be used to refer to the array indices.

For example:

var meuArray = [];
meuArray['asd'] = 4;
console.log(meuArray.asd); // 4

It is perfectly possible to overwrite the behavior of indexOf and make that chunk of code work in an interesting way:

var meuArray = [];
meuArray['indexOf'] = -5;
console.log(meuArray.indexOf <= -1); // true
meuArray['indexOf'] = 5;
console.log(meuArray.indexOf <= -1); // false

Answer 2

campos.indexOf <= -1

By performing this comparison, you are comparing a function ("fields.indexof") with an integer "-1". The comparison is valid because "indexof" is an attribute of the Array object, although it is a function, but could be an integer like the attribute "length".