I should start by noting I am extremely inexperienced with Javascript.

I have a program in javascript that declares a variable right at the start, in global scope, as follows

var psycho_pairs

Later I write to this variable with the following

async function experimentInit() {
  psycho_pairs = [];
  let num_samples = 10;
  let num_repeats = 4;
  let increment = (high_bound - low_bound)/(num_samples);
  for (let i = low_bound   increment/2; i < high_bound; i  = increment){
      let f2_low  = Math.max(i - 1,low_bound);
      let f2_high = Math.min(i   1,high_bound);
      let f2_incr = (f2_high - f2_low)/(num_samples);
      for (let j = f2_low; j < f2_high - f2_incr/2; j  = f2_incr){
          for (let n = 0; n < num_repeats; n  ){
              psycho_pairs.push([i,j]);
          }
      }
  }
  console.log(psycho_pairs);
}

That final line prints what I expect it to, so thats good. Later on in the program I have the following lines of code

function routine_2afc_stimRoutineBegin(snapshot) {
  return async function () {
    console.log(psycho_pairs);
    let rand_ind = Math.floor(Math.random() * psycho_pairs.length)
    var cur_pair = psycho_pairs[rand_ind];
    var psycho_pairs = psycho_pairs.splice(rand_ind,1);
    console.log(cur_pair);
  }
}

In this case the first console.log just prints undefined and the program throws the following error

TypeError: Cannot read properties of undefined (reading 'length')

It seems to me that the variable psycho_pairs is being updated locally within the first function, but that isn't being reflected in the global workspace, so in the second function the variable is still just undefined. However, I thought the var keyword explicitly creates global variables that are mutable inside of functions? What mistake have I made with the initialisation/creation of this variable?

CodePudding user response：

In Javascript var variables have a strange property where they are initialized at the top of the block, and then set wherever you set them. In your second function, you are re-declaring psycho_pairs, bringing a local variable called psycho_pairs into scope, even though the declaration is below your console.log.

You can read more about it here: https://developer.mozilla.org/en-US/docs/Glossary/Hoisting