javascript - Using the conditions value in ifelse - Stack Overflow

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I'm wondering if it's possible to access a condition's value directly like the following example.

var a = ["pear", "kiwi", "orange", "apple"]
if(a.indexOf("orange") !== -1){
  console.log(this) //as a.indexOf("orange") has been evaluated already above this prints 2
}

This would also make ternary operators less bloaty

var a = ["pear", "kiwi", "orange", "apple"]
var b = ((a.indexOf("orange") !== -1) ? this : '') //"this" equals 2

Thanks

EDIT: Clearing this question up for any future visitors. Basically this question is about retrieving the resulting value of what is evaluated in an if/else statement. In the example of

var a = ["pear", "kiwi", "orange", "apple"]
if(a.indexOf("orange") !== -1){ //is basically if(2 !== -1)
  console.log(this) //would then be "2" from the already evaluted a.indexOf from above
}

I'm wondering if it's possible to access a condition's value directly like the following example.

var a = ["pear", "kiwi", "orange", "apple"]
if(a.indexOf("orange") !== -1){
  console.log(this) //as a.indexOf("orange") has been evaluated already above this prints 2
}

This would also make ternary operators less bloaty

var a = ["pear", "kiwi", "orange", "apple"]
var b = ((a.indexOf("orange") !== -1) ? this : '') //"this" equals 2

Thanks

EDIT: Clearing this question up for any future visitors. Basically this question is about retrieving the resulting value of what is evaluated in an if/else statement. In the example of

var a = ["pear", "kiwi", "orange", "apple"]
if(a.indexOf("orange") !== -1){ //is basically if(2 !== -1)
  console.log(this) //would then be "2" from the already evaluted a.indexOf from above
}

• javascript

Share Improve this question Follow edited Dec 30, 2016 at 22:39 Charles asked Dec 30, 2016 at 18:37 CharlesCharles 5311 silver badge44 bronze badges 3

• 1 Note that what you're asking for is not accessing the condition's value. The condition is either true or false. You want to access the value of one of the sub-expressions within the condition (specifically, what indexOf() returns). – Barmar Commented Dec 30, 2016 at 19:42
• @Barmar Hence why the question seems to have confused everyone. It would have made sense to ask about the conditions value, which could work in theory – aw04 Commented Dec 30, 2016 at 19:51
• Which is why one should read all the details in the question, including the ments in the code, to make sense of it. People aren't always very good at finding the right words. – Barmar Commented Dec 30, 2016 at 19:52

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7 Answers 7

Sorted by: Reset to default 6

You can simply store it before the statement if the goal is to not evaluate twice. The answer to your literal question is no.

const orangeIndex = a.indexOf("orange")

if (orangeIndex !== -1) {
  console.log(orangeIndex)
}

Same concept applies to the ternary operator.

As others have shown, you can also declare a variable and do the actual assignment in the if statement itself, but IMO this makes your code less readable without adding any value.

There's no implicit facility, but you can assign the parison value to a variable:

var a = ["pear", "kiwi", "orange", "apple"], result;
if (result = (a.indexOf("orange") !== -1)){
  console.log(result);
}

edit — the same technique can be applied if you just want part of the evaluated expression:

var a = ["pear", "kiwi", "orange", "apple"], result;
if ((result = a.indexOf("orange")) !== -1){
  console.log(result);
}

Now the .indexOf() return value is retained in result instead of the parison result.

If you want really terse code, you can assign a variable within the condition.

var orangeIndex;
var a = ["pear", "kiwi", "orange", "apple"];
if ((orangeIndex = a.indexOf("orange")) !== -1) {
  console.log(orangeIndex);
}

You can also do it in a ternary:

var orangeIndex;
var a = ["pear", "kiwi", "orange", "apple"]
var b = (((orangeIndex = a.indexOf("orange")) !== -1) ? orangeIndex : '');
console.log(b);

In both cases, don't forget the parentheses around the assignment. This is needed because assignment has lower precedence than parison operators, so it would otherwise set the variable to true or false.

Yes, this is in fact very simple. Example

if (yourvariable = yourcondition) {
    //Do something
} else if (yourvariable2 = yourcondition2) {
    //Do something else
} else if ((yourvariable3 = yourcondition3) || true) { //This is an else, but you memorized a condition inside it
    //Do something
}

When you assign a value to a variable, then the variable will hold that value and also, the result of the operator will be the value, therefore

if (foo = bar) {
    //Some operations
}

is equivalent logically to

if (bar) {
    foo = bar;
    //Some operations
}

but it is longer and if bar happens to be a function with a lot of operations, then it is not ideal to evaluate it twice.

This is as close as it gets

var a = ["pear", "kiwi", "orange", "apple"], result;
if ((result = a.indexOf("orange")) !== -1) {
  console.log(result);
}

Here, @result = the index rather than the result of the logical operation that Pointy offered.

I refactor your code. You can hold result of condition in a variable.

var a = ["pear", "kiwi", "orange", "apple"];

var isOrange = a.indexOf("orange") !== -1;

if(isOrange){
  console.log(this) //as a.indexOf("orange") has been evaluated already above this prints 2
}

var a = ["pear", "kiwi", "orange", "apple"]
var b = isOrange ? this : '') //"this" equals 2

You can acplish this through memoization. lodash provides a method to do this called memoize.

Creates a function that memoizes the result of func. If resolver is provided, it determines the cache key for storing the result based on the arguments provided to the memoized function. By default, the first argument provided to the memoized function is used as the map cache key. The func is invoked with the this binding of the memoized function.

// your collection
const fruit = ["pear", "kiwi", "orange", "apple"]

/*
 * a basic indexOf function that we can memoize.
 * @example
 * indexOf(fruit)('kiwi') // 1
 */
const indexOf = list => Array.prototype.indexOf.bind(list)

/*
 * a memoized version of IndexOf that is seeded with the list
 * @example
 * memoizedIndexOfA('kiwi') // 1
 */
const memoizedIndexOfA = _.memoize(indexOf(fruit))

// the first time it is called in the `if` it is calculated
if (memoizedIndexOfA("orange") !== -1){

    // the value was previously calculated, so retrieve from the cache.
    console.log(memoizedIndexOfA("orange"))
}

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