Question

I am very new to JS, have been working in C/C++ before,
I need an equivalent of below C structure in JSON

struct tmp_t{
int a;
char c_str[1024];
};

struct tmp2_t{
int a2;
.
.
char c2_str[1024];
};

struct my {
int number;
struct tmp_t tmp[100];
struct tmp2_t tmp2[100][1000];
};

For a json like

var myJSON = {
"number":0,
.
.
};

I need to access it like

myJSON.tmp[0].a = 10;
myJSON.tmp2[0][1].c2_str = "hello world"

any input is highly appreciated

Answer 1

Javascript properties are not typed like they are in C so there is no purely “equivalent” expression in javascript. You don’t predeclare typed data structures like your C code has. I given variable or property in javascript can be assigned any value or reference – there is not hard typing. So without variables that can only contain a specific type like C has, there’s no pre-declaring of data structure definitions like you have included from C.

Instead, you just declare the properties you want to use on a live object or if you intend to use many of them, you can create a prototype which you can instantiate when needed.

A direct declaration of a live object instance somewhat like your last structure would look like this:

var my = {
    number: 10,
    tmp: new Array(100),
    tmp2: new Array(100)
};

This would declare an object named my that had three properties called number, tmp and tmp2. number initially contained the number 10 and the other two properties contained arrays of length 100 who’s values were undefined. I don’t know of any compact way to predefine your two dimensional array in javascript without running code in a loop to initialize it.

This data defintion would let you access my.number, my.tmp and so on.

If you want your arrays to contains objects with properties themselves, then you need to populate those arrays with the objects.

var my = {
    number: 10,
    tmp: [{a: 1, c_str: "foo"}, {a: 2, c_str: "whatever"}],
    tmp2: new Array(100)
};

Or, in code, you could add in item to the tmp array with code like this:

var my = {
    number: 10,
    tmp: [],
    tmp2: []
};

my.tmp.push({a: 1, c_str: "foo"});
my.tmp.push({a: 2, c_str: "whatever"});

Or, you could create the object separately and then put it in the array:

var obj = {};        // new empty object
obj.a = 1;           // assign property
obj.c_str = "foo";   // assign property
my.tmp.push(obj);    // put object into the array
obj = {};            // new empty bject
obj.a = 2;
obj.c_str = "whatever";
my.tmp.push(obj);

Or, you could assign each property individually like this:

my.tmp.push({});           // put empty object into the array
my.tmp[0].a = 1;           // assign property to the object
my.tmp[0].c_str = "foo";   // assign property to the object
my.tmp.push({});
my.tmp[1].a = 2;
my.tmp[1].c_str = "whatever";

In either case, you could then access the data like this:

console.log(my.tmp[0].a);       // 1
console.log(my.tmp[0].c_str);   // "foo"