Python 的字典 get 方法的 Javascript 等价物是什么

Question

Python's get method for dictionaries lets me specify what should be returned if a key doesn't exist. For my current case I want a dictionary returned. How do I do this in Javascript?

Answer 1

There is no javascript equivalent of the python dictionary get method. If you would write it yourself, as a function, it would look like this:

function get(object, key, default_value) {
    var result = object[key];
    return (typeof result !== "undefined") ? result : default_value;
}

Use it like:

var obj = {"a": 1};
get(obj, "a", 2); // -> 1
get(obj, "b", 2); // -> 2

Note that the requested key will also be found in a prototype of obj.

If you really want a method rather than a function ( obj.get("a", 2) ), you need to extend the prototype of Object. This is generally considered a bad idea though, see Extending Object.prototype JavaScript

Answer 2

JavaScript has no helper feature to do that. You need to test explicitly.

if ("myProperty" in myObject) {
    return { another: "object" };
} else {
    return myObject.myProperty;
}

You can use a ternary operator to do the same thing with less code.

return ("myProperty" in myObject) ? myObject.myProperty : { another: "object" };

Answer 3

You could use a proxy for this (really new ):

var handler = {
    get: function(target, name){
        return name in target?
        target[name] :
        "Default";
    }
};
var dictionary={"hi":true};
var dict = new Proxy(dictionary, handler);
dict.a = 1;
dict.b = undefined;

console.log(dict.a, dict.b,dict.hi); // 1, undefined,true
console.log(dict.new); //"Default"

 //the proxied object gets changed:

console.log(dictionary.a, dictionary.b,dictionary.hi); // 1, undefined,true
console.log(dictionary.new); //undefined

A proxy is an object that reflects all changes and requests trough an handler. In this case we can write/access propertys of dictionary normally, but if we access values that do not exist it'll return "Default"

Answer 4

I prefer to use the logical OR like this:

foo.bar || 'default'

If checks is foo.bar is falsy, so it returns 'default' if bar is undefined.

You just need to care, that foo is an object. Otherwise a ReferenceError is thrown.

Answer 5

this works for me

let obj = {"a": 1};
let default = 100
obj["a"] || default; // -> 1
obj["b"] || default; // -> 100

But! there are some limitation, if !!obj["a"] === false we always get default value... so it's better to just check if key in obj, to be completely sure.

Answer 6

With modern javascript you can use the ?? operator

const result = obj[key] ?? default;

Note that cases like {myKey: undefined} or {myKey: null} will also return the default value, which may or may not be the desired behaviour.