Dart Assignment operators

1. Basic Assignment Operator =

Stores the value of the right-hand expression into the left-hand variable, overwriting any pre-existing value.

Syntax:

variable = value_or_expression;

int age;
age = 10;        // Assign the value 10 to age
String name;
name = "sam"; // String value assignmentCode language: JavaScript (javascript)

2. Null-Aware Assignment Operator ??=

Only assigns the value if the left-hand variable is null. If the variable holds a non-null value, its original content remains unchanged.

Syntax:

variable ??= value;

This shorthand is equivalent to writing:

if (variable == null) {
  variable = value;
}Code language: JavaScript (javascript)

Example:

void main() {
	int? total = null;
	total ??= 5;
	print(total); // Outputs 5 — assignment runs since total was null

	int? count = 100;
	count ??= 200;
	print(count); // Outputs 100 — assignment skipped, count is non-null
}Code language: PHP (php)

3. Compound Assignment Operators

Combines an arithmetic/bitwise operation and assignment into one step

For any operator op:

a op= b is functionally identical to a = a op b

Category	Operator	Description
Basic Arithmetic	+=	Add then assign result
	-=	Subtract then assign result
	*=	Multiply then assign result
	/=	Float division then assign result
Integer Division	~/=	Truncating integer division then assign
Modulo	%=	Remainder division then assign
Bitwise Left Shift	<<=	Left bit shift then assign
Bitwise Right Shift	>>=	Signed right bit shift then assign
Unsigned Right Shift	>>>=	Unsigned right bit shift then assign
Bitwise AND	&=	Bitwise AND then assign
Bitwise XOR	^=	Bitwise XOR then assign
Bitwise OR	|=	Bitwise OR then assign

General rule for compound operators:	a op= b	a = a op b
Sample translation:	a += b	a = a + b

Practical examples:

void main() {
	var num = 2;

	// *= Multiplication compound assignment
	num *= 3; 
	print(num); // 6, equivalent to num = num * 3

	// += Addition
	num += 4;
	print(num); // 10, equivalent to num = num + 4

	// -= Subtraction
	num -= 2;
	print(num); // 8

	// /= Floating-point division
	double d = 10;
	d /= 2;
	print(d); // 5.0

	// ~/= Integer truncating division
	int n = 7;
	n ~/= 2;
	print(n); // 3

	// %= Modulo remainder
	int m = 9;
	m %= 2;
	print(m); // 1

	// <<= Left bit shift
	int bit1 = 1;
	bit1 <<= 2; // 1 << 2 = 4
	print(bit1); // 4

	// >>= Signed right bit shift
	int bit2 = 8;
	bit2 >>= 2; // 8 >> 2 = 2
	print(bit2); // 2

	// >>>= Unsigned right bit shift
	int bit3 = -8;
	bit3 >>>= 2;
	print(bit3); //4611686018427387902, a very large positive number 

	// &= Bitwise AND
	int x = 6; // Binary 110
	x &= 3;    // Binary 011
	print(x);  // Result binary 010 = decimal 2

	// ^= Bitwise XOR
	int y = 5;
	y ^= 3;
	print(y);

	// |= Bitwise OR
	int z = 4;
	z |= 1;
	print(z); // 5
}Code language: PHP (php)

Let’s break down the left shift example: int bit1 = 1; shifted left by 2 positions.

The 32-bit binary representation of decimal 1 is:

00000000 00000000 00000000 00000001

x << n: Shifts all bits of x left by n positions. Empty low-order bits on the right fill with 0, and overflow high-order bits on the left get discarded entirely.

After shifting left twice, the binary becomes:

00000000 00000000 00000000 00000100

This binary value translates to decimal 4.

We will cover logical operators in a later section. For now, focus on mastering these assignment operators — they are the core takeaway of this lesson.

void main() {
  // 1. Basic assignment =
  var a = 2;
  print(a == 2);

  // 2. Multiplication compound assignment *=
  a *= 3; // Equivalent to a =  a * 3;
  print(a == 6);

  // 3. Addition compound assignment +=
  a += 4; // Equivalent to a = a + 4;
  print(a == 10);

  // 4. Null-aware assignment ??=
  int? b; // Declared nullable int, defaults to null
  b ??= 20;
  print(b == 20);

  b ??= 99; // b is no longer null; value remains unchanged
  print(b == 20);

  // 5. Subtraction compound assignment -=
  a -= 5;
  print(a == 5);

  // 6. Modulo compound assignment %=
  a %= 3;
  print(a == 2);
}Code language: Dart (dart)

Every print statement above will output true