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Declaration statement

Overview

Declaration statement is the statement to declare variables.

var a; // initializing 'a' to null

Declaration with initializer

Declaration of a variable has initializer to initialize it. If there is no initializer, the variable should be null automatically.

var a = 10; // initializing 'a' to 10 as integer.

Multiple declaration of variables

Multiple variables can be put in one declaration statement separated by ,. Each variable can have its own initializer, or can have no initializer.

var a, b;
var x = 10, y;

Constant value

Variable can be constant by declaration with const.

const a = 10; // initializing 'a' to 10 as integer and it is constant value.

Constant value can not be modified and it will be compile error when trying to change that variable's value.

const a = 10;
a = 20; // error.

Destructuring assignment

Destructuring assignment is supported and a spread/rest operator is also available. But a spread/rest operator of destructuring assignment for Object is not supported.

var [a, ...b] = [10, 20, 30];
// => a == 10
// => b == [20, 30]

Of course, assignment is rejected when it declared as const.

const [a, ...b] = [10, 20, 30];
a = 10; // error.

You can use the style of an array or object in assignment. The following three styles are available.

  • Array Style ... each item in the array will be assigned to a variable in the order.
  • Object Key Style ... the value according to a key name will be assigned to the variable of the same name as the key.
  • Object Style ... each value will be assigned to the variable bound to each key.

Here is an example.

var [a, b, , ...c] = [1, 2, 3, 4, 5, 6];  // 3rd parameter is skipped.
var { x, y } = { x: 20, y: { a: 30, b: 300 } };
var { x: d, y: { a: e, b: f } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("f = ", f);
System.println("x = ", x);
System.println("y = ", y);

Here is the result.

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
f = 300
x = 20
y = {"a":30,"b":300}

Pattern Matching

When it is an declaration, the pattern matching is available. If a part of variables is a literal, it will be checked if the same value. And if matching a pattern is failed, the exdeption of NoMatchingPatternException will be raised.

Here is an example.

var [a, b, , ...c] = [1, 2, 3, 4, 5, 6];
var { x, y } = { x: 20, y: { a: 30, b: 300 } };
var { x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("x = ", x);
System.println("y = ", y);

// => .y.b requires 300, but it is 3 in actual.
var { x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 3 } };

Here is the result.

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
x = 20
y = {"a":30,"b":300}
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at <main-block>(test.kx:14)
Range in Pattern Matching

If you write a Range in a pattern, it will check inside that Range.

Here is an example.

var [1..10, b] = [1, 100];  // b => 100
var [1..10, c] = [2, 100];  // c => 100
var [1..10, d] = [9, 100];  // d => 100
var [1..10, e] = [11, 100]; // NoMatchingPatternException will occur.

Examples

Example 1. Normal case

Code

var a;
System.println(a.isUndefined ? "null" : "defined");

Result

null

Example 2. With initializer

Code

var a = 5;
System.println(a);

Result

5

Example 3. With initializer of expression

Code

function pow8(n) {
    return n ** 8;
}

var a = "2 ** 8 = " + pow8(2);
System.println(a);

Result

2 ** 8 = 256

Example 4. Multiple variable declaration

Code

var a, b = 10, c, d = "abc";
System.println(a.isUndefined ? "null" : a);
System.println(b.isUndefined ? "null" : b);
System.println(c.isUndefined ? "null" : c);
System.println(d.isUndefined ? "null" : d);

Result

null
10
null
abc

Example 5. Constant value (1)

Code

const a = 10;
a = 20;

Result

Error: Can not assign a value to the 'const' variable near the <test.kx>:2

Example 6. Constant value (2)

Code

If you did not initialize a constant variable, you can initialize it only one time by assignment.

const a;
a = 20;
System.println(a);

Result

20

Example 7. Constant value (3)

Code

It should be error when initializing twice.

const a;
a = 20;
a = 30; // error.

Result

Error: Can not assign a value to the 'const' variable near the <test.kx>:3

Example 7. Constant value (4)

Code

Even if it is Object, it should be compile error.

const a = { x: 10 };
a = { y: 20 };

Result

Error: Can not assign a value to the 'const' variable near the <test.kx>:2

Example 6. Constant value (5)

Code

But the object member can be modified.

const a = { x: 10 };
a.x = 20;
System.println(a.x);

Result

20

Example 7. Destructuring assignment (1)

Code

var [a, ...b] = [10, 20, 30];
System.println(a);
System.println(b[1]);

Result

10
30

Example 7. Destructuring assignment (2)

Code

const [a, ...b] = [10, 20, 30];
a = 10;
b = 20;

Result

Error: Can not assign a value to the 'const' variable near the <test.kx>:2
Error: Can not assign a value to the 'const' variable near the <test.kx>:3

Example 8. Array & Object Style

Code

var [a, b, , ...c] = [1, 2, 3, 4, 5, 6];
var { x, y } = { x: 20, y: { a: 30, b: 300 } };
var { x: d, y: { a: e, b: f } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("f = ", f);
System.println("x = ", x);
System.println("y = ", y);

Result

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
f = 300
x = 20
y = {"a":30,"b":300}

Example 9. Pattern Matching

Code

var [a, b, , ...c] = [1, 2, 3, 4, 5, 6];
var { x, y } = { x: 20, y: { a: 30, b: 300 } };
var { x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("x = ", x);
System.println("y = ", y);

// => .y.b requires 300, but it is 3 in actual.
var { x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 3 } };

Result

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
x = 20
y = {"a":30,"b":300}
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at <main-block>(test.kx:14)

Example 10. Range in Pattern

Code

If you write a Range in a pattern, it will check inside that Range.

Here is an example.

var [1..10, b] = [1, 100];  // b => 100
var [1..10, c] = [2, 100];  // c => 100
var [1..10, d] = [9, 100];  // d => 100
System.println(b);
System.println(c);
System.println(d);

var [1..10, e] = [11, 100]; // NoMatchingPatternException will occur.

Result

100
100
100
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at <main-block>(test.kx:8)