The 3 Different Equals

The 3 different equals

You have = the assignment operator, == the 'equal' comparison operator and === the 'identical' comparison operator.

$a = $b     Assign      Sets $a to be equal to $b.
$a == $b    Equal       TRUE if $a is equal to $b.
$a === $b   Identical   TRUE if $a is equal to $b, and they are of the same type. (introduced in PHP 4)

For more info on the need for == and ===, and situations to use each, look at the docs.

Three equals signs in php comparison

The easiest way to see it, is that == checks equality and === checks identicality. Equality will check the value, but identicality will check the variable type too.

Examples:

var_dump('true' == true); // bool(true)
var_dump('true' === true); // bool(false)

Triple (3) Equal Signs

Triple equal sign in javascript means equality without type coercion.

For example:

1=="1"     // true, automatic type coersion
1==="1"    // false, not the same type.

Difference between != and !==

= is an assignment operator, e.g. If you run var x = 1; then x will have the value of 1.

== (or !=) is a comparison operator that checks if the value of something is equal to the value of something else. e.g. if(x == 1) will evaluate to true and so will if(x == true) because 1 will evaluate to true and 0 evaluate to false.

=== (or !==) is another comparison operator that checks if the value of something is equal to the value of, and is the same type as something else. e.g. if(x === 1) will evaluate to true however, if(x === true) will evaluate to false because 1 (the value of x) is an integer and true is a boolean.

Why use triple-equal (===) in TypeScript?

Imagine you're designing TypeScript from scratch. Essentially, you're trying to optimize for making safer code easier to write (TypeScript design goal 1) with a few caveats which prevent you from doing everything you'd like.

JavaScript compatibility (TypeScript design goal 7)

JavaScript should be valid Typescript with no changes.

CoffeeScript makes no guarantees regarding this, so it can convert all instances of == to === and simply tell users don't rely on =='s behavior. TypeScript cannot redefine == without breaking all JavaScript code that relies on its behavior (despite this having sad implications for 3).

This also implies that TypeScript cannot change the functionality of === to, for example, check the types of both operands at compile time and reject programs comparing variables of different types.

Further, compatibility is not limited to simply JavaScript programs; breaking compatibility also affects JavaScript programmers by breaking their assumptions about the differences between == and ===. See TypeScript non-goal number 7:

Introduce behaviour that is likely to surprise users. Instead have due consideration for patterns adopted by other commonly-used languages.

JavaScript as the target of compilation (TypeScript design goal 4)

All TypeScript must be representable in JavaScript. Further, it should be idiomatic JavaScript where possible.

Really though, the TypeScript compiler could use methods returning booleans for all comparisons, doing away with == and === entirely. This might even be safer for users: define a type-safe equality method on each TypeScript type (rather like C++ operator==, just without overloading).

So there is a workaround (for users comparing classes). unknown or any variables can have their types narrowed before using the type-safe equality method.

Which to prefer

Use === everywhere you would in JavaScript. This has the advantage of avoiding the pitfalls common to ==, and doesn't require you to maintain an additional method. The output of the TypeScript compiler will be close to idiomatic JavaScript. Using == has very much the same pitfalls as JavaScript, particularly when you have any, [], or {} involved. As an exception, using == null to check for null or undefined may save headaches if library code is inconsistent.

A method for reference equality (behavior like === for classes) could be confused with a deep/value recursive equality check. Furthermore, === is widely used in TypeScript, and making your code fall in line with conventions is usually more important than any small bit of type safety.

What is exactly the meaning of === in javascript?

'===' means equality without type coersion. In other words, if using the triple equals, the values must be equal in type as well.

e.g.

0==false   // true
0===false  // false, because they are of a different type
1=="1"     // true, auto type coersion
1==="1"    // false, because they are of a different type

Source: http://longgoldenears.blogspot.com/2007/09/triple-equals-in-javascript.html

How do the PHP equality (== double equals) and identity (=== triple equals) comparison operators differ?

Difference between `==` and `===`

The difference between the loosely == equal operator and the strict === identical operator is exactly explained in the manual:

Comparison Operators


┌──────────┬───────────┬───────────────────────────────────────────────────────────┐
│ Example  │ Name      │ Result                                                    │
├──────────┼───────────┼───────────────────────────────────────────────────────────┤
│$a ==  $b │ Equal     │ TRUE if $a is equal to $b after type juggling.            │
│$a === $b │ Identical │ TRUE if $a is equal to $b, and they are of the same type. │
└──────────┴───────────┴───────────────────────────────────────────────────────────┘

Loosely `==` equal comparison

If you are using the == operator, or any other comparison operator which uses loosely comparison such as !=, <> or ==, you always have to look at the context to see what, where and why something gets converted to understand what is going on.

Converting rules

Converting to boolean
Converting to integer
Converting to float
Converting to string
Converting to array
Converting to object
Converting to resource
Converting to NULL

Type comparison table

As reference and example you can see the comparison table in the manual:

Loose comparisons with ==

┌─────────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬─────────┬───────┬───────┐
│         │ TRUE  │ FALSE │   1   │   0   │  -1   │  "1"  │  "0"  │ "-1"  │ NULL  │ array() │ "php" │  ""   │
├─────────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼─────────┼───────┼───────┤
│ TRUE    │ TRUE  │ FALSE │ TRUE  │ FALSE │ TRUE  │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE   │ TRUE  │ FALSE │
│ FALSE   │ FALSE │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ TRUE  │ TRUE    │ FALSE │ TRUE  │
│ 1       │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ 0       │ FALSE │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ TRUE  │ FALSE   │ TRUE  │ TRUE  │
│ -1      │ TRUE  │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE   │ FALSE │ FALSE │
│ "1"     │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ "0"     │ FALSE │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ "-1"    │ TRUE  │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE   │ FALSE │ FALSE │
│ NULL    │ FALSE │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ TRUE    │ FALSE │ TRUE  │
│ array() │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ TRUE    │ FALSE │ FALSE │
│ "php"   │ TRUE  │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ TRUE  │ FALSE │
│ ""      │ FALSE │ TRUE  │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE   │ FALSE │ TRUE  │
└─────────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴─────────┴───────┴───────┘

Strict `===` identical comparison

If you are using the === operator, or any other comparison operator which uses strict comparison such as !== or ===, then you can always be sure that the types won't magically change, because there will be no converting going on. So with strict comparison the type and value have to be the same, not only the value.

Type comparison table

As reference and example you can see the comparison table in the manual:

Strict comparisons with ===

┌─────────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┬─────────┬───────┬───────┐
│         │ TRUE  │ FALSE │   1   │   0   │  -1   │  "1"  │  "0"  │ "-1"  │ NULL  │ array() │ "php" │  ""   │
├─────────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼─────────┼───────┼───────┤
│ TRUE    │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ FALSE   │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ 1       │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ 0       │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ -1      │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ "1"     │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ "0"     │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE │ FALSE   │ FALSE │ FALSE │
│ "-1"    │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE │ FALSE   │ FALSE │ FALSE │
│ NULL    │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE  │ FALSE   │ FALSE │ FALSE │
│ array() │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ TRUE    │ FALSE │ FALSE │
│ "php"   │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ TRUE  │ FALSE │
│ ""      │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE │ FALSE   │ FALSE │ TRUE  │
└─────────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴─────────┴───────┴───────┘

Difference between == and === in JavaScript

=== and !== are strict comparison operators:

JavaScript has both strict and
type-converting equality comparison.
For strict equality the objects being
compared must have the same type and:
Two strings are strictly equal when they have the same sequence of
characters, same length, and same
characters in corresponding positions.
Two numbers are strictly equal when they are numerically equal (have
the same number value). NaN is not
equal to anything, including NaN.
Positive and negative zeros are equal
to one another.
Two Boolean operands are strictly equal if both are true or
both are false.
Two objects are strictly equal if they refer to the same Object.
Null and Undefined types are == (but not ===). [I.e. (Null==Undefined) is true but (Null===Undefined) is false]

Comparison Operators - MDC

The 3 Different Equals