What's the secret behind generating a crisp 400 on paper?

Understanding Why it Prints 400 I'm struggling to comprehend the logic behind this

var x = {},
      y = { key: "y" },
      z = { key: "z" };
      x[y] = 100;
      x[z] = 200;
    
    console.log(x[y] + x[z]);

Answer №1

When using bracket notation in JavaScript, it is important to note that object keys must be either strings or symbols. If a non-string/non-symbol is used as a key, it will automatically be converted into a string.

In the given scenario, both variables b and c are objects. When these objects are converted into strings, they result in [object Object]. This means that referencing a[b] and a[c] actually points to the same property within the object. The most recent assignment made was:

a[c] = 200;

Therefore, whenever a[b] or a[c] is accessed, the value returned will be 200. Hence, evaluating the expression:

a[b] + a[c]

yields

200 + 200

due to the fact that both b and c get converted into the same string for use as a property key.

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