> ## Documentation Index
> Fetch the complete documentation index at: https://docs.near.org/llms.txt
> Use this file to discover all available pages before exploring further.
# The Standard
> Learn how Non-Fungible Tokens (NFT) are defined on NEAR
In contrast with fungible tokens, non-fungible tokens (NFT) are unitary and therefore unique. This makes NFTs ideal to represent ownership of assets such as a piece of digital content, or a ticket for an event.
As with fungible tokens, NFTs are **not stored** in the user's wallet, instead, each NFT lives in a **NFT contract**. The NFT contract works as a bookkeeper, being in charge of creating, storing and transferring NFTs.
**NFT & Marketplaces**
Be mindful of not confusing an NFT with an NFT-marketplace. NFT simply store information (metadata), while NFT-marketplaces are contracts where NFT can be listed and exchanged for a price.
***
## NEP-171 (NFT Interface)
[NEP-171](https://github.com/near/NEPs/tree/master/neps/nep-0171.md) is the blueprint for all non-fungible tokens on NEAR. It defines a **common set of rules** and **functions** that the contract MUST implement to be considered a non-fungible token contract.
Notice that the NEP-171 defines the **interface** and **expected behavior** of a non-fungible token contract, but it does not dictate how the internal logic should be implemented
Different NFT contracts can have different internal implementations while still adhering to the NEP-171 standard
### Interface
#### `nft_token` (*read-only*)
Returns the token information for a given `token_id`
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_token(token_id: string): { token_id: string, owner_id: string } | null
```
#### `nft_transfer`
Transfers the `token_id` from the caller to the `receiver_id`, optionally the function can include a `memo` field to provide additional information to the contract
The caller must be the **either** the current owner of the token or an **account** that has been **approved to transfer** the token on behalf of the owner, such as a marketplace contract. The approval mechanism is defined in the [NEP-178 standard](https://github.com/near/NEPs/blob/master/neps/nep-0178.md).
> *Requirement: The caller must attach [exactly 1 yoctoNEAR](../../smart-contracts/security/one_yocto) to the call*
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_transfer(receiver_id: string, token_id: string, approval_id?: number, memo: string?): void
```
#### `nft_transfer_call`
The function transfers the `token_id` to the `receiver_id` **and calls the method `nft_on_transfer(sender_id, previous_owner_id, token_id, msg)`** on `receiver_id`.
Optionally the function can include a `memo` for the NFT contract, and a `msg` field to which will be sent to the receiver contract.
> 📖 This function is useful to transfer NFTs to a contract and trigger some action on the receiver side in a single transaction, thus acting as **attaching NFTs to a function call**
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_transfer_call(receiver_id: string, token_id: string, approval_id?: number, memo?: string, msg: string): Promise {}
```
Smart contracts expecting to **receive** Non-Fungible Tokens **must** implement this method.
The method **must** return a boolean value indicating whether the token should be returned to the sender (`true`) or not (`false`).
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_on_transfer(sender_id: string, previous_owner_id: string, token_id: string, msg: string): boolean
```
⚠️ Note that this method does not need to be implemented by the NFT contract itself, but rather by any contract that expects to receive non-fungible tokens
#### `nft_resolve_transfer`
This method is used as a [callback](../../smart-contracts/anatomy/crosscontract#callback-function) to resolve the `nft_transfer_call` transaction, handling refunds if necessary.
It must return `true` if the token was successfully transferred to `receiver_id`, or `false` if the token was returned to `owner_id`.
```js theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_resolve_transfer(owner_id: string, receiver_id: string, token_id: string, approved_account_ids?: Record): boolean
```
***
## NEP-177 (NFT Metadata)
[NEP-177](https://github.com/near/NEPs/blob/master/neps/nep-0177.md) is an extension to the NEP-171 standard that defines the **metadata** for both non-fungible tokens and non-fungible token contracts.
Metadata provides **key information** , such as the contract's **name** or the NFT's **title**. Particularly, the following fields MUST be included in the smart contract and token's metadata:
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
type NFTContractMetadata = {
spec: string, // required, essentially a version like "nft-1.0.0"
name: string, // required, ex. "Mochi Rising — Digital Edition" or "Metaverse 3"
symbol: string, // required, ex. "MOCHI"
icon: string|null, // Data URL
base_uri: string|null, // Centralized gateway known to have reliable access to decentralized storage assets referenced by `reference` or `media` URLs
reference: string|null, // URL to a JSON file with more info
reference_hash: string|null, // Base64-encoded sha256 hash of JSON from reference field. Required if `reference` is included.
}
type TokenMetadata = {
title: string|null, // ex. "Arch Nemesis: Mail Carrier" or "Parcel #5055"
description: string|null, // free-form description
media: string|null, // URL to associated media, preferably to decentralized, content-addressed storage
media_hash: string|null, // Base64-encoded sha256 hash of content referenced by the `media` field. Required if `media` is included.
copies: number|null, // number of copies of this set of metadata in existence when token was minted.
issued_at: number|null, // When token was issued or minted, Unix epoch in milliseconds
expires_at: number|null, // When token expires, Unix epoch in milliseconds
starts_at: number|null, // When token starts being valid, Unix epoch in milliseconds
updated_at: number|null, // When token was last updated, Unix epoch in milliseconds
extra: string|null, // anything extra the NFT wants to store on-chain. Can be stringified JSON.
reference: string|null, // URL to an off-chain JSON file with more info.
reference_hash: string|null // Base64-encoded sha256 hash of JSON from reference field. Required if `reference` is included.
}
```
***
## NEP-178 (NFT Approval Management)
[NEP-178](https://github.com/near/NEPs/blob/master/neps/nep-0178.md) is an extension to the NEP-171 standard that defines the **approval management** for non-fungible tokens.
The approval mechanism allows the owner of an NFT to authorize another account (for example, a marketplace contract) to transfer the token on their behalf.
### Interface
#### `nft_is_approved` (*read-only*)
Returns whether an `approved_account_id` is actually approved to transfer the `token_id` on behalf of the token owner. If `approval_id` is provided, it will also check that the approval ID matches.
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_is_approved(token_id: string, approved_account_id: string, approval_id?: number): boolean
```
#### `nft_approve`
Grants approval to `account_id` to transfer the `token_id` on behalf of the token owner. Optionally, the function can include a `msg` field to provide additional information to the contract.
> *Requirement: The caller must attach a deposit of at least `1 yoctoNEAR` plus the storage cost for adding the new approval*
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_approve(token_id: string, account_id: string, msg?: string): void
```
#### `nft_revoke`
Revokes approval for `account_id` to transfer the `token_id` on behalf of the token owner.
> *Requirement: The caller must attach [exactly 1 yoctoNEAR](../../smart-contracts/security/one_yocto) to the call*
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_revoke(token_id: string, account_id: string): void
```
#### `nft_revoke_all`
Revokes all approvals for the `token_id`.
> *Requirement: The caller must attach [exactly 1 yoctoNEAR](../../smart-contracts/security/one_yocto) to the call*
```ts theme={"theme":{"light":"github-light","dark":"github-dark"}}
nft_revoke_all(token_id: string): void
```