Question

What's its value?

const colorConfig = {
  red: true,
  blue: false,
  green: true,
  black: true,
  yellow: false,
};

const colors = ['pink', 'red', 'blue'];

console.log(colorConfig.colors[1]);

• A: true
• B: false
• C: undefined
• D: TypeError

Answer 1

Answer: D

In JavaScript, we have two ways to access properties on an object: bracket notation, or dot notation. In this example, we use dot notation (colorConfig.colors) instead of bracket notation (colorConfig["colors"]).

With dot notation, JavaScript tries to find the property on the object with that exact name. In this example, JavaScript tries to find a property called colors on the colorConfig object. There is no property called colors, so this returns undefined. Then, we try to access the value of the first element by using [1]. We cannot do this on a value that's undefined, so it throws a TypeError: Cannot read property '1' of undefined.

JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket [ and keeps going until it finds the closing bracket ]. Only then, it will evaluate the statement. If we would've used colorConfig[colors[1]], it would have returned the value of the red property on the colorConfig object.