How to use reduce to evaluate truthy, mixed, and falsey values for a collection that's either an array or object

Question

For arrays like [ true , 1, 'car', { a: '1' }] how can I use reduce to evaluate that the entire array is true or false? It has to consider truthy and falsey values.

Moreover, using reduce, if all values are truthy return true. If the elements are mixed with truthy and falsey values return false. If the elements are all falsey return false.

I also cannot use every. I am actually trying to implement the every method by using reduce as an assignment. It works it just doesn't pass for all-truthy results nor does it cast the result to a bool bonus question :) if I have

_.every([1], _.identity)).to.be.true;
_.every([0], _.identity)).to.be.false;

Also is {} truthy? bonus bonus question

Here's my code but it doesn't use reduce which is probably making it harder to evaluate the truthy, falsey and mixed values. Thanks for your help!

    _.every = function(collection, iterator) {
      // TIP: Try re-using reduce() here.
      var results = [];
      if ( iterator === undefined ) {
        _.each( collection ,  function( item ) { 
          results.push( item );
        });
      } else { 
        if ( Array.isArray( collection ) === true ) {
          var copiedArr = collection.slice();
          // iterate through each item using each
          _.each( collection , function( item ) { 
          // input our own iterator calling truth test
          if ( iterator( item ) === true ) { 
        // store results in array
        results.push( true )
          } else {
        results.push( false );
          }
        } );
      } else if ( typeof collection === 'object' ) {
        var copiedObj = JSON.parse(JSON.stringify( collection ));

        // iterate through each item using each
        _.each( copiedObj , function( item ) { 

            // input our own iterator calling truth test
            if ( iterator( item ) === true ) { 
              // store results in array
              results.push( true )
            } else {
              results.push( false );
            }
          });
        }
      } // end of iterator defined else

      // if array contains false return false
      if ( _.contains( results , false) ) {
        return false
      } else {
      // else return true
        return true;
      }
    } // end of _.every
    ```

Answer 1

It sounds like you want to check whether all values nested anywhere in the structure are truthy. You can use a recursive function to check whether each value is truthy, coercing to boolean with Boolean, or if a value is an object, use .reduce and iterate over its .values (which will work for both objects and arrays). The reduce callback can be

(a, b) => a && Boolean(checkTruthy(b))

which will return the previous value accumulator if it was false, else it will return the truthyness of the current item being iterated over.

const checkTruthy = param =>
  typeof param !== 'object'
  ? Boolean(param)
  : Object.values(param)
      .reduce(
        (a, b) => a && Boolean(checkTruthy(b)),
        true
      );


console.log(checkTruthy([ true , 1, 'car', { a: '1' }]));
console.log(checkTruthy([ true , 1, 'car', { a: '1', b: 0 }]));
console.log(checkTruthy([ true , 1, 'car', { a:  0,  b: '1' }]));

Or, to be a bit more efficient but possibly less readable (YMMV), you can move the Boolean outside the .reduce:

const checkTruthy = param => Boolean(
  typeof param !== 'object'
  ? param
  : Object.values(param)
      .reduce(
        (a, b) => a && checkTruthy(b),
        true
      )
);


console.log(checkTruthy([ true , 1, 'car', { a: '1' }]));
console.log(checkTruthy([ true , 1, 'car', { a: '1', b: 0 }]));
console.log(checkTruthy([ true , 1, 'car', { a:  0,  b: '1' }]));

Or, if you don't need recursion, it's much simpler:

const checkTruthy = obj => Boolean(
  Object.values(obj)
    .reduce((a, b) => a && b)
);


console.log(checkTruthy([ true , 1, 'car', { a: '1' }]));
console.log(checkTruthy([ true , 1, 'car', true, { a: '1' }]));
console.log(checkTruthy([ true , 1, 'car', false, { a: '1' }]));

Answer 2

To implement every, you will want to use reduce with the && operation as the reducer and true as the initial value. On every value, call the predicate function, and - if you don't trust it - cast its result to a boolean:

function every(arr, predicate) {
    return arr.reduce((acc, val) => acc && Boolean(predicate(val)), true);
}

If you need your every to work on "object collections" as well, not just arrays, you probably have written your own version of reduce already that works on arbitrary collections:

function every(collection, predicate) {
    return reduce(collection, (acc, val) => acc && Boolean(predicate(val)), true);
}