# @ohos.util.Stack (Linear Container Stack)

Stack is implemented based on the array data structure. It follows the principle Last Out First In (LOFI) and supports data insertion and removal at one end.

Unlike [Queue](js-apis-queue.md), which is implemented based on the queue data structure and supports insertion at one end and removal at the other end, Stack supports insertion and removal at the same end.

**Recommended use case**: Use Stack in LOFI scenarios.

This topic uses the following to identify the use of generics:
- T: Type

> **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.


## Modules to Import

```ts
import { Stack } from '@kit.ArkTS';
```

## Stack

### Properties

**Atomic service API**: This API can be used in atomic services since API version 12.

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

| Name| Type| Readable| Writable| Description|
| -------- | -------- | -------- | -------- | -------- |
| length | number | Yes| No| Number of elements in a Stack.|


### constructor

constructor()

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

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200012 | The Stack's constructor cannot be directly invoked. |

**Example**

```ts
let stack : Stack<number | string | Object> = new Stack();
```


### push

push(item: T): T

Adds an element at the top of this Stack.

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| T | Element added.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The push method cannot be bound. |

**Example**

```
class C1 {
  name: string = ""
  age: string = ""
}
let stack : Stack<number | string | C1> = new Stack();
let result = stack.push("a");
let result1 = stack.push(1);
let c : C1  = {name : "Dylan", age : "13"};
let result2 = stack.push(c);
```

### pop

pop(): T

Removes the top element from this Stack.

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Return value**

| Type| Description|
| -------- | -------- |
| T | Element removed. If the Stack is empty, **undefined** is returned.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The pop method cannot be bound. |

**Example**

```ts
let stack : Stack<number> = new Stack();
stack.push(2);
stack.push(4);
stack.push(5);
stack.push(2);
stack.push(4);
let result = stack.pop();
```

### peek

peek(): T

Obtains the top element of this Stack.

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Return value**

| Type| Description|
| -------- | -------- |
| T | Element obtained.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The peek method cannot be bound. |

**Example**

```ts
let stack : Stack<number> = new Stack();
stack.push(2);
stack.push(4);
stack.push(5);
stack.push(2);
let result = stack.peek();
```

### locate

locate(element: T): number

Obtains the index of the first occurrence of the specified element in this Stack.

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Parameters**

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

**Return value**

| Type| Description|
| -------- | -------- |
| number | Index of the element. If no match is found, **-1** is returned.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The locate method cannot be bound. |

**Example**

```ts
let stack : Stack<number> = new Stack();
stack.push(2);
stack.push(4);
stack.push(5);
stack.push(2);
let result = stack.locate(2);
```

### forEach

forEach(callbackFn: (value: T, index?: number, stack?: Stack&lt;T&gt;) => void,
thisArg?: Object): void

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

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Parameters**

| Name| Type| Mandatory| Description|
| -------- | -------- | -------- | -------- |
| callbackFn | function | Yes| Callback invoked to traverse the elements in the Stack.|
| thisArg | Object | No| Value of **this** to use when **callbackFn** is invoked. The default value is this instance.|

callbackFn parameters

| 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. The default value is **0**.|
| stack | Stack&lt;T&gt; | No| Instance that calls the **forEach** API. The default value is this instance.|

**Error codes**

For details about the error codes, see [Universal Error Codes](../errorcode-universal.md) and [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 401      | Parameter error. Possible causes: 1. Mandatory parameters are left unspecified; 2. Incorrect parameter types. |
| 10200011 | The forEach method cannot be bound. |

**Example**

```ts
let stack : Stack<number> = new Stack();
stack.push(2);
stack.push(4);
stack.push(5);
stack.push(4);
stack.forEach((value : number, index ?: number) :void => {
  console.log("value:" + value, "index:" + index);
});
```

### isEmpty

isEmpty(): boolean

Checks whether this Stack is empty (contains no elements).

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Return value**

| Type| Description|
| -------- | -------- |
| boolean | Returns **true** if the Stack is empty; returns **false** otherwise.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The isEmpty method cannot be bound. |

**Example**

```ts
let stack : Stack<number> = new Stack();
stack.push(2);
stack.push(4);
stack.push(5);
stack.push(4);
let result = stack.isEmpty();
```

### [Symbol.iterator]

[Symbol.iterator]\(): IterableIterator&lt;T&gt;

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

**Atomic service API**: This API can be used in atomic services since API version 12.

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

**Return value**

| Type| Description|
| -------- | -------- |
| IterableIterator&lt;T&gt; | Iterator obtained.|

**Error codes**

For details about the error codes, see [Utils Error Codes](errorcode-utils.md).

| ID| Error Message|
| -------- | -------- |
| 10200011 | The Symbol.iterator method cannot be bound. |

**Example**
```ts
let stack : Stack<number> = new Stack();
stack.push(2);
stack.push(4);
stack.push(5);
stack.push(4);

// Method 1:
while(!stack.isEmpty()) {
  // Service logic
  let item = stack.pop();
  console.log("value:" + item);
}

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