Filter / Reduce

filter(func(x,i))

• filter() returns a subset of an array based on a function you provide. This function should be a predicate (returning true or false) which the filter() will use to build the collection it returns. The function will be passed two values: the first is the value of the current element; the other is the current index.

let nums = [1,2,3,4,5];

// filter only the even values
nums.filter(function(x) {
    return x % 2 === 0;
}); // [ 2, 4 ]

// filter only even indices
nums.filter(function(x,i) {
    return i % 2 === 0;
}); // [ 1, 3, 5 ]

let names = [
  { fname : 'Dario', lname : 'Argento' },
  { fname : 'George', lname : 'Romero' },
  { fname : 'Sam', lname : 'Raimi' },
  { fname : 'Eli', lname : 'Roth' },
  { fname : 'George', lname : 'Lucas' }
];

names.filter(function(x,i) {
    return x.fname === 'George';
});
/*
[ { fname: 'George', lname: 'Romero' },
  { fname: 'George', lname: 'Lucas' } ]
*/

reduce(func(acc,currentValue)[, startingAccumulatorValue])

• reduce() is used to combine all of the values within an array into a single value. The reduction function can take as many as two arguments. The function is provided two values, which you will perform the combination operation on.
• acc is the accumulated total of the running operation.

• currentValue is the value at each index.

• reduce() will use the first index of the array as its starting accumulator value by default. If you would like to use an alternative starting value, you may optionally provide a second argument after the function, which will be the starting accumulator value.

let nums = [1,2,3,4,5];

nums.reduce(function(acc,currentValue) {
    return acc + currentValue;
}); // 15

nums.reduce(function(acc,currentValue) {
    return acc * currentValue;
}); // 120

let mixNums = [1,5,2,9,54,23];

// Return the largest number in the array
mixNums.reduce(function(acc,currentValue) {
    return (acc > currentValue) ? acc : currentValue;
})

let values = [1,5,'kiwi', 'geoff',true];

// Sum all of the strictly number values in an array
values.reduce(function(acc,currentValue){
    acc = (typeof acc === 'number') ? acc : 0;
    currentValue = (typeof currentValue === 'number') ? currentValue : 0;
    return acc + currentValue;
}); // 6

Skill Drill

1. Create an array of numbers named nums that stores the values 1-10.

2. Use filter() to return only the numbers from your nums array that are even.

3. Use reduce() to sum all of the numbers in your nums array.

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