A major use case of Ethereum wallets is to acquire and track assets. This is often done by pre-loading a list of approved assets, which is insecure, or by stepping through a tedious process of adding each asset. The RPC method wallet_watchAsset allows users to easily track new assets with a suggestion from the sites they visit.

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Calling wallet_watchAsset

Calling wallet_watchAsset method returns true/false if the asset is accepted/denied by the user, or an error if there is something wrong with the request. Unlike other methods, the default definition of wallet_watchAsset params is not an array, but to keep it compatible with code conventions of other apps, wallet-sdk supports both array and object format. 
interface WatchAssetParameters {
  type: string; // The asset's interface, e.g. 'ERC20'
  options: {
    address: string; // The hexadecimal Ethereum address of the token contract
    symbol?: string; // A ticker symbol or shorthand, up to 5 alphanumerical characters
    decimals?: number; // The number of asset decimals
    image?: string; // A string url of the token logo
  };
}
Here’s an example of how to suggest the client wallet add a custom token: 
function onApproveWatchAsset() {
  // your approval callback implementation
}

function onDenyWatchAsset() {
  // your denying callback implementation
}

function onError(message: string) {
  // your error callback implementation
}

// Use wallet_watchAsset
ethereum
  .request({
    method: "wallet_watchAsset",
    params: {
      type: "ERC20",
      options: {
        address: "0xcf664087a5bb0237a0bad6742852ec6c8d69a27a",
        symbol: "WONE",
        decimals: 18,
        image: "https://s2.coinmarketcap.com/static/img/coins/64x64/11696.png",
      },
    },
  })
  .then((response) => {
    const result: boolean = response as boolean;
    result ? onApproveWatchAsset() : onDenyWatchAsset();
  })
  .catch((err) => onError(err.message));