# Linear Container Queue

> **NOTE**
>
> The initial APIs of this module are supported since API version 8. Newly added APIs will be marked with a superscript to indicate their earliest API version.

**Queue** follows the principle of First In First Out (FIFO). It supports insertion of elements at the end and removal from the front of the queue. **Queue** is implemented based on the queue data structure.

Unlike **[Deque](js-apis-deque.md)**, which supports insertion and removal at both the ends, **Queue** supports insertion at one end and removal at the other end.

**Recommended use case**: Use **Queue** in FIFO scenarios.

## Modules to Import

```ts
import Queue from '@ohos.util.Queue';  
```


## Queue

### Attributes

**System capability**: SystemCapability.Utils.Lang

| Name| Type| Readable| Writable| Description|
| -------- | -------- | -------- | -------- | -------- |
| length | number | Yes| No| Number of elements in a queue (called container later).|


### constructor

constructor()

A constructor used to create a **Queue** instance.

**System capability**: SystemCapability.Utils.Lang

**Example**

```ts
let queue = new Queue();
```


### add

add(element: T): boolean

Adds an element at the end of this container.

**System capability**: SystemCapability.Utils.Lang

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| element | T | Yes| Target element.|

**Return value**

| Type| Description|
| -------- | -------- |
| boolean | Returns **true** if the element is added successfully; returns **false** otherwise.|

**Example**

```ts
let queue = new Queue();
let result = queue.add("a");
let result1 = queue.add(1);
queue.add(1);
let b = [1, 2, 3];
queue.add(b);
let c = {name : "lala", age : "13"};
let result3 = queue.add(c);
```

### pop

pop(): T

Removes the first element from this container.

**System capability**: SystemCapability.Utils.Lang

**Return value**

| Type| Description|
| -------- | -------- |
| T | Element removed.|

**Example**

```ts
let queue = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(2);
queue.add(4);
let result = queue.pop();
```

### getFirst

getFirst(): T

Obtains the first element of this container.

**System capability**: SystemCapability.Utils.Lang

**Parameters**

| Type| Description|
| -------- | -------- |
| T | The first element obtained.|

**Example**

```ts
let queue = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(2);
let result = queue.getFirst();
```

### forEach

forEach(callbackfn: (value: T, index?: number, Queue?: Queue<T>) => void,
thisArg?: Object): void

Uses a callback to traverse the elements in this container and obtain their position indexes.

**System capability**: SystemCapability.Utils.Lang

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| callbackfn | function | Yes| Callback invoked to traverse the elements in the container.|
| thisArg | Object | No| Value to use when the callback is invoked.|

callbackfn

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| value | T | Yes| Value of the element that is currently traversed.|
| index | number | No| Position index of the element that is currently traversed.|
| Queue | Queue<T> | No| Instance that invokes the **forEach** method.|

**Example**

```ts
let queue = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(4);
queue.forEach((value, index) => {
  console.log("value:" + value, index);
});

```

### [Symbol.iterator]

[Symbol.iterator]\(): IterableIterator<T>

Obtains an iterator, each item of which is a JavaScript object.

**System capability**: SystemCapability.Utils.Lang

**Return value**

| Type| Description|
| -------- | -------- |
| IterableIterator<T> | Iterator obtained.|

**Example**
```ts
let queue = new Queue();
queue.add(2);
queue.add(4);
queue.add(5);
queue.add(4);

// Method 1:
for (let item of queue) { 
  console.log("value:" + item); 
}

// Method 2:
let iter = queue[Symbol.iterator]();
let temp = iter.next().value;
while(temp != undefined) {
  console.log("value:" + temp);
  temp = iter.next().value;
}
```