Here's a big one. JavaScript has two ways to check if things are equal:
===is strict equality. It checks that both the value and the type are the same.==is loose equality. It converts the values to the same type first (using that sneaky type coercion from Grade 2), and then compares.
And there are two matching "not equal" operators: !== (strict) and != (loose).
Watch what happens:
console.log(5 === 5);
console.log(5 === "5");
console.log(5 == "5");
console.log(0 == false);
console.log("" == 0);
console.log(null == undefined);
console.log(null === undefined);It prints
true false true true true true false
With ==, JavaScript thinks the number 5 equals the string "5", that 0 equals false, and that an empty string equals zero. These loose conversions follow a long list of complicated rules that almost nobody remembers.
The loose == operator causes so many bugs that most developers simply never use it. Always use === and !==. They do exactly what you'd expect: two things are equal only if they're the same type and the same value. No surprises, no coercion, no Buggsy.
= assigns a value (put into the box). === compares values (are they the same?). Writing = when you mean === is one of the most common beginner mistakes. One equals sign is an action; three equals signs is a question.