Little utility D3 functions

D3 code before

// x and y are scale functions
var line = d3.svg.line()
  .x(function (d) { return x(new Date(d.date)); })
  .y(function (d) { return y(+d.y); });

This is very common D3 callback code. Here is the same code with callbacks refactored with d3-helpers

var line = d3.svg.line()
  .x(d3h('date', d3h.newDate, x))
  .y(d3h('y', Number, y));

Notice several benefits:

1. The .x callback is easier to read from left to right: grab property date, then call d3h.newDate function on it, then call x function. No more inside out composition flow as in x(new Date(d.date))
2. Allowing only property names and functions to apply makes the author's intention clear. +d.y is always ambiguous: did the author forget to add something or was this the intention? Writing d3h('y', Number, ...) makes it explicit.
3. By eliminating writing each callback function, we eliminate potential sources of errors. In addition, since every function passed as argument is external, they becoming testable.

Node:

npm install d3-helpers --save
var d3h = require('d3-helpers');

Browser:

bower install d3-helpers
<script src="bower_components/d3-helpers/index.js"></script>
// attaches as window.d3h object

// index
d3h (alias d3h.d)
d3h.i
d3h.noop
d3h.undef
d3h.pass (alias d3.datum)
d3h.property
d3h.yes / no
d3h.index
d3h.value
d3h.newDate
d3h.hermit

d3-helpers is a well-tested function augmented by other tiny functions. First the d3h function itself

Returns a function that can chain property access and function composition.

d3h('propertyName', fnToApply, 'method name to call', 'anotherPropertyName', orAnotherFn, ...);

var foo = {
  getName: function() { return this.name; },
  name: 'foo'
};
function concatSelf(x) { return x + x; }
function add2(x) { return x + 2; }
var f = d3h('getName', concatSelf, 'length', add2);
f(foo) // returns 8

// f is the same as
function (obj) {
  return add2(concatSelf(obj.getName()).length);
}

Use on d argument:

  .x(d3h('length', xScale));
  // d3h.d is an alias
  .(d3h.d('length', xScale));

When calling a method on the object, this is bound to the object, and it is passed itself as first argument.

Same chaining as d3h.d but operates on the second argument, usually the index

.y(d3h.i(yScale))
// same as
.y(function (d, i) {
  return yScale(i);
});

If you want to return the index element, you need to execute the function to create the callback

.y(d3h.i())
// same as
.y(function (d, i) {
  return i;
});

Same as function () {}

Same as function () { return; } if you need a function that always returns undefined.

Same as function (d) { return d; }

You can pass a function as argument, then it works as a wrapped function for any value passed next. For example to scale by function triple we can replace

function triple(x) { return 3 * x; }
.attr('left', function (d) { return triple(d); })
// with
.attr('left', d3h.pass(triple));

function property(name) {
  return function (obj) {
    return obj[name];
  };
}
// if passed function(s) as additional arguments
function property(name, f, g, ...) {
  return function (obj) {
    return g(f(obj[name]));
  };
}

Logically, read this from left to right. First grab named property, then apply function f, then apply to the result g, etc.

Useful to extract a property and convert type, for example for D3 selections

.text(d3h.property('name'))
.width(d3h.property('age', Number))
.text(d3h.property('date', String))

or scale property from datum

var x = d3.time.scale(), y = d3.scale.linear();
var line = d3.svg.line()
  .x(function (d) { return x(d.date); })
  .y(function (d) { return y(d.y); });
// same using d3-helpers
var line = d3.svg.line()
  .x(d3h.property('date', x))
  .y(d3h.property('y', y))

d3h.yes function always returns true, d3h.no function always returns false.

Same as function (d, i) { return i; }

Same as

function (val) {
  return function () {
    return val;
  };
}

Value could be anything: number, string, undefined, even another function.

Same as function (d) { return new Date(d); } to get around the new keyword requirement in JavaScript for Dates. Useful for constructing Date instances inside d3h.property

var x = d3.time.scale();
var line = d3.svg.line()
  .x(function (d) { return x(new Date(d.date)); })
// same using d3-helpers
var line = d3.svg.line()
  .x(d3h.property('date', d3h.newDate, x))

Wraps a function that should not receive any arguments. Returned function just calls the original without any arguments.

function failIfArguments() {
  if (arguments.length) {
    throw new Error('I cannot handle arguments');
  }
  return 42;
}
d3h.hermit(failIfArguments)(100); // 42

I have another tiny library with single exported function called functional-pipeline. It is very similar to d3h function for building a left to right pipelines, but has better debug mode.

Author: Gleb Bahmutov © 2014

• @bahmutov
• glebbahmutov.com
• blog

License: MIT - do anything with the code, but don't blame me if it does not work.

Spread the word: tweet, star on github, etc.

Support: if you find any problems with this module, email / tweet / open issue on Github

Copyright (c) 2014 Gleb Bahmutov

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.