Why putting if condition inside first bracket doesnot work while using ternary operator?

Question

if condition inside first bracket causes error:

function fd(t) {
  let output = '';
  if (t >= 41) {
    let mns = 45
    (mns === 1) ? (output = "true") : (output = "false");
  }
  return output;
}

console.log(fd(65));

Error message: "ReferenceError: can't access lexical declaration `mns' before initialization"

if condition without first bracket works just fine:

function fd(t) {
  let output = '';
  if (t >= 41) {
    let mns = 45
    mns === 1 ? (output = "true") : (output = "false");
  }
  return output;
}

console.log(fd(65));

Can anyone please explain me this behavior?

By the way: use just `output = String(mns === 1);` instead of the conditional. Or just skip the stringification and return a Boolean to begin with, with `output = mns === 1;`. — Sebastian Simon, Jul 31 '18 at 13:32

Unmitigated · Answer 1 · 2018-08-01T23:50:03.957

2

You are missing a semicolon after let mns = 45 to conclude the statement. Without the semicolon to conclude the statement, it would be like trying to pass (mns === 1) as an argument to the let statement. It would be like the two lines were combined into one: let mns = 45(mns === 1) ? (output = "true") : (output = "false");

The second function works because there is no bracket around msn === 1, so it won't try to pass it as an argument to the let statement.

As a general rule, always remember to add a semicolon after each JavaScript statement.

function fd(t) {
  let output = '';
  if (t >= 41) {
    let mns = 45;
    (mns === 1) ? (output = "true") : (output = "false");
  }
  return output;
}

console.log(fd(65));

As Xufox suggested in the comments, you could just move the ternary operator to the return statement to avoid ambiguity.

function fd(t) {
     let mns;
      if (t >= 41) {
        mns = 45;
      }
      return (mns === 1) ? ("true") : ("false");
}

console.log(fd(65));

edited Aug 01 '18 at 23:50

answered Jul 31 '18 at 13:19

Unmitigated

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the why second function works? – noobie Jul 31 '18 at 13:20
1

It’s not trying to pass `(mns === 1) ? (output = "true") : (output = "false")` as an argument, only `mns === 1`. The entire statement would read `let mns = 45(mns === 1) ? (output = "true") : (output = "false");`. – Sebastian Simon Jul 31 '18 at 13:22
@noobie Because there's no bracket. – Unmitigated Jul 31 '18 at 13:22
1

@noobie Because that one is _unambiguous_ in terms of [Automatic Semicolon Insertion](https://stackoverflow.com/q/2846283/4642212). – Sebastian Simon Jul 31 '18 at 13:22
@Xufox I have edited my answer. – Unmitigated Jul 31 '18 at 13:28
@hev1 Oh, the alternative line to the conditional was actually meant to be directed towards the OP, not you. – Sebastian Simon Jul 31 '18 at 13:31

score 1 · Answer 2 · edited Aug 01 '18 at 23:49

1

That happens because let declaration is not concluded yet. To end the declaration use ; or declare as var instead of let.

function fd(t) {
  let output = '';
  if (t >= 41) {
    let mns = 45;  // note the semicolan or declare it as var
    (mns === 1) ? (output = "true") : (output = "false");
  }
  return output;
}

console.log(fd(65));

edited Aug 01 '18 at 23:49

Unmitigated

76,500
11
62
80

answered Jul 31 '18 at 13:19

BlackBeard

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Why putting if condition inside first bracket doesnot work while using ternary operator?

2 Answers2