Github page for source code in this article:
https://github.com/toreaurstadboss/JsLinqSimpleProjection
https://www.npmjs.com/package/jslinqsimpleprojection
Npm package for source code in this article:
Since I started working with Linq in C#, I missed a good way of doing much of the same functionality in Javascript. Today, there are several Linq libraries for Javascript and Typescript, and libraries such as Backbone.Js or Lodash also containing a lot of
helpful operators or utility methods. As an educational exercise, I was looking into a simple way of doing a projection method in pure Javascript (no es6 syntax). Here is what I made.
First off, we need to be able to project an array of objects by listing up properties. In ES6 Javascript we could use arrow functions. But I wanted to support pure Javascript. So I choose to use a comma separated list of property or field values to dive into the
array object, written in Json notation of course.
Consider first this array as an example:
var someCountries = [
{ country: "Norway", population: 5.2, code: "NO" },
{ country: "Finland", population: 5.5, code: "SU" },
{ country: "Iceland", population: 0.4, code: "IC" },
{ country: "Sweden", population: 10.2, code: "SW" }
];
We want to project this array using a new method select on the array of which we use the
Array.prototype to achieve. Note that this will immediately add methods to all array objects immediately in the global scope.
Now consider a projection of just the 'country' and the 'population' fields of the Json structure.
Given a method call of just these two properties, we want to create a select projection method.
First consider this lightweight linqmodule implementation, using an IFE (Immediately invoked function expression) and using the revealing module pattern.
We expose the method dump to this module.
var linqmodule = (function() {
projection = function(members) {
var membersArray = members.replace(/s/g, "").split(",");
var projectedObj = {};
for (var i = 0; i < this.length; i++) {
for (var j = 0; j < membersArray.length; j++) {
var key = membersArray[j];
if (j === 0) {
projectedObj[i] = {};
}
projectedObj[i][key] = this[i][key];
}
}
return projectedObj;
};
Array.prototype.select = projection;
dumpmethod = function(arrayobj) {
var result = "";
result += "[";
for (var i = 0; i < Object.keys(arrayobj).length; i++) {
var membersArray = Object.keys(arrayobj[i]);
for (var j = 0; j < membersArray.length; j++) {
if (j === 0) {
result += "{";
}
var key = membersArray[j];
result +=
"key: " +
key +
" , value: " +
arrayobj[i][key] +
(j < membersArray.length - 1 ? " , " : "");
if (j === membersArray.length - 1) {
result +=
"}" + (i < Object.keys(arrayobj).length - 1 ? "," : "") + "\n";
}
}
}
result += "]";
return result;
};
return {
dump: dumpmethod
};
})();
Now it is easy to dump the contents of our projected array (which is copied into a new object) using the dump method:
result = someNums.select("country,population");
document.getElementById("result").innerText = linqmodule.dump(result);
console.log(result);
Note that our new object contains only the country and population fields in the Json structure, not the code. We have created a simple projection mechanism in Javascript in a self contained module!
