The document discusses the functions provided by the Underscore.js library. It covers basic functions like filter, map, and reduce. It also covers more advanced functions like zip, union, and difference. Finally, it provides examples of chaining functions together and using Underscore in live examples like calculating averages and modeling the birthday paradox problem.

1. _
Underscore.js
provides 80-odd functions that support both the usual functional suspects
Andries Nieuwenhuize - Quadrant Software B.V. - 12 maart 2013

2. Inhoud
 Basic
 Filter
 Projectie
 Fold
 Advanced
 Zip
 Distinct
 Extend
 Live
 Chaining
 Sudoku
 Column-row invert
 Birthday paradox

3. _.filter([1, 2, 3], function(num) {
Basics return num % 2 == 1;
});
-> [1, 3]
Filter
Projection
_.reject([1, 2, 3], function(num) {
Fold return num % 2 == 1;
});
-> [2]
_.without([1, 2, 3, 4, 5, 6, 7, 8, 9], 8, 1);
-> [2, 3, 4, 5, 6, 7, 9]
_.where(persons, { age : 22 });
-> [{ name: “Fred”, age : 22, userId: 001 },
{ name: “Hans”, age : 22, userId: 002 }]

4. [{ name: “Fred”, age : 22, userId: 001 },
Basics { name: “Hans”, age : 22, userId: 002 }
{ name: “Karel”, age : 67, userId: 006 }]
Filter
_.map([1, 2, 3], function(num) {
Projection return num * num;
Fold });
-> [1, 4, 9]
_.pluck(persons, “name” );
-> [ “Fred”, “Hans”, “Karel” ]
_.pick(persons[0], “name”, “age” );
-> { name: “Fred”, age : 22 }
_.omit(persons[0], “name”, “age” );
-> { userId: 001 }

5. Basics _.reduce([1, 2, 3], function(memo, num) {
return memo * num;
Filter }, 1);
Projection -> 9
Fold
_.reduce([1, 2, 3], function(memo, num) {
return memo + num;
}, 0);
-> 6
_.max([1, 2, 3]);
-> 3

6. Advanced
Zip
Distinct e.d. _.zip(['-', '-', '-'], ['+', '+', '+'])
Extend -> ['-', '+', '-', '+', '-', '+']
_.object(['a', 'b', 'c'], [1, 2, 3]);
-> {'a' : 1, 'b' : 2, 'c' : 3 }

7. Advanced
_.union([1, 2, 3], [101, 2, 1, 10], [2, 1]);
Zip -> [1, 2, 3, 101, 10]
Distinct e.d.
_.intersection(
Extend
[1, 2, 3], [101, 2, 1, 10], [2, 1]);
-> [1, 2]
_.difference([1, 2, 3, 4, 5], [5, 2, 10]);
-> [1, 3, 4]
_.uniq([1, 2, 1, 3, 1, 4]);
-> [1, 2, 3, 4]

8. var sum = function(lst) {
Advanced return _.reduce(lst, function(memo, num) {
return memo + num;
} , 0);
Zip
};
Distinct e.d. > sum([1, 2, 3]);
Extend
_.mixin({
sum : function(lst) {
return _.reduce(lst, function(memo, num) {
return memo + num;
} , 0);
},
avg : function(lst) {
return _.sum(lst) / lst.length;
}
});
> _.avg([1, 2, 3]);

9. Live
var persons = [
{ name: "Piet",
Chaining
age: 22 },
Sudoku { name: "Kees",
Column-row invert age : 54 },
Birthday paradox { name: "Sarah",
age : 18 }
];
_.chain(persons)
.pluck("age")
.avg()
.value();
-> 31,333333333333333332

10. Live
Chaining
Sudoku
Column-row invert
Birthday paradox
// First step in Sudoku-algorithm (reduce)
_.chain([1, 2, 3, 4, 5, 6, 7, 8, 9])
.without(7, 4, 3, 9) // block
.without(3, 9) // row
.without(7, 5, 9, 3) // column
.value();
-> [1,2,6,8]

11. // Medieval is Evil!
var invert = function(columns) {
var res = [];
Live for(var i = 0; i< columns.length; i++) {
var row = columns[i];
Chaining for (var j = 0; j < row.length; j++) {
if(! res[j])
Sudoku res[j] = [];
Column-row invert
res[j][i] = row[j];
Birthday paradox }
}
return res;
};
// Just zip-it
_.zip(['moe', 'larry', 'curly'],
[30, 40, 50],
[true, false, false]);
-> [moe,30,true],
[larry,40,false],
[curly,50,false]

12. var min = 1, // 1th January
max = 365; // 31 December
Live
// produce birthdays
var getNextBirthday = function() {
Chaining return _.random(min, max);
Sudoku };
Column-row invert // check for double dates
Birthday paradox var isMatchFound = function(lst) {
return lst.length > _.uniq(lst).length;
};
// Birthday paradox (BP) - probability theorie // loop until first hit
// Some have the same brithday: var getFirstHit = function() {
// 100% - 366 people needed. -> 365 in 'normal' var loop = function(lst) {
year lst.push(getNextBirthday());
// BP -> 99% 57 people if (IsMatchFound(lst)) {
return lst.length; // attempts
// BP -> 50% 23 people
}
else { return loop(lst); }
};
return loop([]); // start
};