Update Variable Dynamically JS

Question

I want to update a value of one field using values of other fields with spaces. If value is null, space shouldn't be added. What logic should I use as I can't use IF in value set.

EXAMPLE

document.getElementById("target").value= Constant+if(name){" "+this.value}+if(age){" "+this.value}+if(height){" "+this.value};

I am using required field so at the end it won't make a difference just want to find a way to learn. — Odin
– Odin, Commented Sep 8, 2014 at 15:20
You can't use an if statement inside an expression. You are looking for the conditional operator. — Felix Kling
– Felix Kling, Commented Sep 8, 2014 at 15:21
I know and I just meant the logic. I said I can't use IF already. — Odin
– Odin, Commented Sep 8, 2014 at 15:36
If you are interested, there are frameworks out there that can do this type of thing for you through binding. Checkout knockout.js, angular.js, backbone.js — Preston S
– Preston S, Commented Sep 8, 2014 at 16:31

Preston S · Accepted Answer · 2014-09-08 16:01:08Z

1

Put your string formatting code in a function where you can use if statements.

document.getElementById("target").value = formatValue();

function formatValue(){
    var formattedString = Constant;

    if(name){
        formattedString += " " + name;
    }

    if(age){
        formattedString += " " + age:
    }

    if(height){
        formattedString += " " + height:
    }

    return formattedString;
}

If you prefer not to have a named function, use a self-invoking anonymous function:

document.getElementById("target").value = (function(){
    var formattedString = Constant;

    if(name){
        formattedString += " " + name;
    }

    if(age){
        formattedString += " " + age:
    }

    if(height){
        formattedString += " " + height:
    }

    return formattedString;
})();

edited Sep 8, 2014 at 16:01

answered Sep 8, 2014 at 15:46

Preston S

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2 Comments

Odin Over a year ago

I know you can use another function for formatting ;) That wasn't the point tho. Thanks anyway!

Preston S Over a year ago

@Odin You can use a self-invoking anonymous function instead.

Brad · Accepted Answer · 2014-09-08 15:21:14Z

1

What you're looking for is called a ternary operator. A boolean condition followed by ? with the true value to return and after : the false value to return.

console.log((name ? name : '') + (age ? age : ''));

For what you're doing though, I wouldn't concatenate a bunch of ternaries in this way. Make an array and then join.

var parts = [];
if (name) {
  parts.push(name);
}
if (age) {
  parts.push(age);
}
// etc. etc.

console.log(parts.join(' '));

If you can use Array.filter (newer browsers), you can simplify your array, not needing all the if statements: http://www.devign.me/javascript-tip-remove-falsy-items-out-of-an-array

answered Sep 8, 2014 at 15:21

Brad

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2 Comments

Odin Over a year ago

I am stuck in the same problem as before, that I have extra spaces. For example if name is not there I will have constant+' '+' '+age+' '+height.

Brad Over a year ago

@Odin Not if you use the join method. And, you can always add spaces after the variables above. I just didn't for simplicity so I could explain a ternary to you.

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