javascript - Why does console.log say undefined, and then the correct value?

Question

This question already has answers here:

Answer 1

The console will print the result of evaluating an expression. The result of evaluating console.log() is undefined since console.log does not explicitly return something. It has the side effect of printing to the console.

You can observe the same behaviour with many expressions:

> var x = 1;
undefined;

A variable declaration does not produce a value so again undefined is printed to the console.

As a counter-example, expressions containing mathematical operators do produce a value which is printed to the console instead of undefined:

> 2 + 2;
4