展开三元语句或切换到if-else

Question

I have the following which works fine with two conditions:

var value = this.value === "foo" ? function(d) {
    return d.foo;
} : function(d) {
    return d.bar;
};

What am I doing wrong when I try to expand the above to the following using if-else ?

var value = 
if (this.value === "foo") {
        function(d) {
            return d.foo;
        };
    } else
if (this.value === "bar") {
    function(d) {
        return d.bar;
    };
} else {
    function(d) {
        return d.qux;
    };
}

I'm happy to stick with operators or switching to if-else : whatever is more readable for me.

Answer 1

if cannot be used on the right-hand side of an assignment statement (which is part of why we have the conditional operator).

Instead, assign in each branch:

var value;
if (this.value === "foo") {
    value = function(d) {
        return d.foo;
    };
} else if (this.value === "bar") {
    value = function(d) {
        return d.bar;
    };
} else {
    value = function(d) {
        return d.qux;
    };
}

---

Although note that that particular code can be simplified:

var key = this.value == "foo" || this.value == "bar" ? this.value : "qux";
var value = function(d) {
    return d[key];
};

Answer 2

You can't use a conditional block statement to assign a value to a variable. You can use the ternary operator you were using in the first example and double it:

var value = (this.value === "foo") ? function(d) {
    return d.foo;
} : (this.value === "bar") ? function(d) {
    return d.bar;
} : function(d) {
    return d.qux;
};

Answer 3

The first once works because the ternary operator by default returns the result of either first or second expression based on the condition

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Conditional_Operator

while the if-else block only check for the condition and do not return anything by default and thats why can not be on the right side of any assignment.

Answer 4

There's another approach. It doesn't quite fit these requirements, but I do assume that these requirements aren't your precise ones (or do you have actual needs for 'foo', 'bar', and 'quz' :-) ? )

You can capture the value in a wrapping function:

function createValueFunc(val) {
    return function(d) {
        return d[val];
    };
}

value = createValueFunc(this.value);

This does not handle your else case, so it may be useless to you. But if it's useful, you can simplify it in ES6 to:

createValueFunc = (name) => (obj) => obj[name];

You'll find such a function in many utility libraries. Underscore calls it property . Ramda calls it prop . Sometimes I've seen it called get as well.