Javascript，如何检查对象是否存在

Question

I am making a script in Javascript script that gets a SQL response, then processes it. Basically, I want to check if the username value exists in result[1] . When it checks, it errors out and says that it does not exist. If it does not exist, I want it to return false, not stop the program.

Here is the code:

if (result[1].username != undefined) {
    return true;
} else {
    return false;
}

I have tried using typeof(result1) == undefined , but it gives me the same error.

Answer 1

You could use the in operator . For example:

 let trueObj = { username: 'Foo' }; let falseObj = { }; if ('username' in trueObj) { console.log('username found in trueObj'); } else { console.log('username not found in trueObj') } if ('username' in falseObj) { console.log('username found in falseObj'); } else { console.log('username not found in falseObj') } 

Answer 2

First, you have to make sure the result exists, otherwise you'd be indexing into undefined which would crash your application.

Second, you can make that check less verbose with:

return (result[1] && result[1].username)

which will return a falsey value if it doesn't exist, and whatever the username is, if it does.

In case you need an explicit true to be what the function returns, you can coerce it:

return (result[1] && (result[1].username && true))

I would make sure to refactor for readability, but that's the gist.

Answer 3

First of all please check whether the result itself exists or not and make the corresponding & operator and i think this will definitely help

if (result && result[1] && result[1].username) {
    return true;
} else {
    return false;
}

But if you don't want to make your code complex then you can try lodash library. https://lodash.com/