Difference Between == and === : A Simple Comparison

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本文解释了JavaScript中===和==运算符之间的区别，并通过示例展示了它们在不同情况下的行为。

Beginners in JavaScript may be confused with the two equal operators, === and ==. So what is the difference between them?

Consider the following example:

console.log( false == "" );
console.log( false === "" );
console.log( 5 == "5" );
console.log( 5 === "5" );

The first and third statements use the == operator so true is printed to the console. The other two statements give false as the results. So Why?

Actually the two operators behave almost the same. Just that the === operator prohibitstype conversion of operands. So the twooperands of the === operator need to be exactly the same (both the value and the type) to give a result of true.

Douglas Crockford gives the following advice in his famous book, JavaScript: The Good Parts: (and I agree with him)

The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable.

...

The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use ===and !==. All of the comparisons just shown produce false with the === operator.