`=` 和 `==` 操作符和 `===` 有什么区别？ （单、双、三等号）

Question

I have written some code and in certain places == is required and in others = is required. Can someone explain the differences or point me in the direction of the resource that can?

Example:

if($("#block").css.display == "none"){
  $("#block").css.display = "block";
}

The only thing I can come up with is that in one I'm changing and in the other I'm checking. But in both I am referring to equality.

Answer 1

= is the assignment operator. It sets a variable (the left-hand side) to a value (the right-hand side). The result is the value on the right-hand side.

== is the comparison operator. It will only return true if both values are equivalent after coercing their types to the same type.

=== is a more strict comparison operator often called the identity operator. It will only return true if both the type and value of the operands are the same.

I would check out CodeCademy for a quick intro to JavaScript.

If you prefer to read more, MDN is a great intro as well.

For those concerned about the source of the term "identity operator" jbabey pointed out that JavaScript: The Definitive Guide seems to mention it.

Answer 2

= assigns a value to a variable

== checks if the two parameter are equal to each other

=== checks if the two parameters are equal to each other and if their type is the same

---

! not operator

!= checks if the two parameters are not equal to each other

!== checks if the two parameters are not equal to each other or the type is not the same

---

one more

> checks if one parameter is greater than the other

>= checks if one parameter is greater than or equal to the other

>== DOESN'T EXIST

---

etcetera...

Answer 3

== is used to test if the value on the left is equal to the value on the right.

= is used to assign the value on the right to the variable on the left.

Answer 4

In javascript you have also the ===.

= This is for set the value to the variable.

== This is for compare if the value is the same.

=== This is for compare if the value is the same and also the type is the same.

Answer 5

The = operator is an assignment operator. You are assigning an object to a value. The == operator is a conditional equality operation. You are confirming whether two things have equal values. There is also a === operator. This compares not only value, but also type.

Assignment Operators

Comparison Operators