问题
I started learning ethereum and web3js and noticed some functions on Web3js are asynchronous. What i want to achieve is get the account balance of a wallet and use the data for something else. My code below
function getAccountBalance2(address){
var wei, balance
//address = document.getElementById("addy").value
return new Promise(function(resolve, reject){
web3.eth.getBalance(address, function(error, wei){
if(error){
console.log("Error with address");
}else{
var balance = web3.fromWei(wei, "ether");
var bal = resolve(balance);
//console.log(bal);
console.log(balance.toNumber());
return balance.toNumber();
}
});
});
}
and i am trying to use the returned value in this function below
function interTransfer(){
var from, to, amount, fromWallet, toWallet
from = document.getElementById("from").value
to = document.getElementById("to").value
amount = document.getElementById("amount").value
if(isWalletValid(from) && isWalletValid(to)){
fromWallet = getAccountBalance2(from);
toWallet = getAccountBalance2(to);
}else{
console.log("Something is wrong")
}
console.log(fromWallet + " "+ toWallet)
}
The output
How do i get the actual value and use it in the interTransfer()
function
回答1:
You need to await the promised values. You can do this with another then
call, and -- to avoid one request having to wait for the previous one to finish -- Promise.all
:
function interTransfer(){
// ...
promise = Promise.all([getAccountBalance2(from), getAccountBalance2(to)])
.then(function ([fromWallet, toWallet]) {
console.log('from wallet', fromWallet, 'to wallet', toWallet);
});
// ...
return promise; // the caller will also need to await this if it needs the values
}
Or, with an async
function and the await
keyword:
function async interTransfer(){
// ...
[fromWallet, toWallet] =
await Promise.all([getAccountBalance2(from), getAccountBalance2(to)]);
console.log('from wallet', fromWallet, 'to wallet', toWallet);
// ...
return [fromWallet, toWallet]; // caller's promise now resolves with these values
}
Note that the return
in the getBalance
callback is useless, and you should probably call reject
with a reason in the case of if(error)
.
来源:https://stackoverflow.com/questions/47601214/how-to-get-a-value-from-a-promise-object