Using a String / text input field as a condition in a JavaScript if statement

Question

I am trying to allow for a if statement condition to be placed into a text field and then for Javascript to take that text field and return if it would be true or false.

logicStr = document.getElementById('logicString').value;

if(logicStr){
alert("true");
}else{
alert("false"); 
}

with the example above if the input was "1 == 0" the alert should be false. But Javascript is seeing it as me asking is the string not null and it isnt so it alerts true. I need to somehow convert to a logic boolean statement.

Stefan Kendall · Accepted Answer · 2011-09-06 15:33:08Z

2

So long as this is all driven by user input and retained locally, eval should work for you.

if(eval(logicStr)){
   alert("true");
}else{
   alert("false"); 
}

answered Sep 6, 2011 at 15:33

Stefan Kendall

68.2k69 gold badges259 silver badges409 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Bengel · Accepted Answer · 2011-09-06 15:36:38Z

0

Look into the JavaScript eval() function. It takes a string input and will evaluate it as a JavaScript expression.

eval("1==0");
false

eval("1==1"):
true

answered Sep 6, 2011 at 15:36

Bengel

1,0537 silver badges12 bronze badges

Comments

Joe · Accepted Answer · 2011-09-06 15:55:08Z

0

Demo

'false === false'.replace(/([a-zA-Z0-9]+) ?(==={0,1}) ?([a-zA-Z0-9]+)/, function (str,b,c,d) {
   if (/false/.test(b)) b = false;
   if (/false/.test(d)) d = false;
   if (/true/.test(b)) b = true;
   if (/true/.test(d)) d = true;

   if (c == '===') return +b === +d;
   else return d == b;
}); // true

'1 == 0'.replace(/([a-zA-Z0-9]+) ?(==={0,1}) ?([a-zA-Z0-9]+)/, function (str,b,c,d) {
   if (/false/.test(b)) b = false;
   if (/false/.test(d)) d = false;
   if (/true/.test(b)) b = true;
   if (/true/.test(d)) d = true;

   if (c == '===') return +b === +d;
   else return d == b;
}); // false

attempt to parse it.

edited Sep 6, 2011 at 15:55

answered Sep 6, 2011 at 15:43

Joe

82.8k18 gold badges130 silver badges147 bronze badges

Collectives™ on Stack Overflow

Using a String / text input field as a condition in a JavaScript if statement

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related