Strict

A quick review of the new features introduced with ECMAScript version 5.

Specifically with reference to null as well as undeclared and undefined data types.

These new features are invoked by specifying 'use strict' with node.js.

Rationale

I often like to experiment with language features, for instance here are my experiments with Golang.

In this repo I am trying out some things with node.js.

Method

We will compare and contrast two mainly identical pieces of code, one with 'use strict' and the other without:

$ diff -uw strict.js not_strict.js 
--- strict.js	2020-02-08 15:43:28.560093898 -0500
+++ not_strict.js	2020-02-08 15:35:35.486426106 -0500
@@ -1,5 +1,3 @@
-"use strict";
-
 // --- null ---
 
 let a = null;
@@ -34,7 +32,7 @@
 // $ node -c strict.js
 // SyntaxError: Delete of an unqualified identifier in strict mode.
 
-//delete g;
+delete g;
 
 // 'undefined'
 console.log("'g' is of type: " + (typeof g));
$

Note that the line 'delete g;' is commented-out in strict.js.

node --check

[This can also be abbreviated as node -c.]

This can be useful for catching errors, but it is really just a syntax check.

For instance:

$ node -c strict.js
/home/owner/Documents/node.js/Strict/strict.js:37
delete g;
       ^

SyntaxError: Delete of an unqualified identifier in strict mode.
    at new Script (vm.js:84:7)
    at checkSyntax (internal/main/check_syntax.js:78:3)
    at internal/main/check_syntax.js:42:3
$

It will not catch runtime errors.

without 'use strict'

Here we will explore null, undeclared and undefined data types.

$ node not_strict.js
null
'a' is of type: object
'null' is of type: object
a is null, 'a' === null ? = true
undefined
'b' is of type: undefined
b is undefined, 'b' === undefined ? = true
I am a Global variable
'g' is of type: string
'g' is of type: undefined
$

[Data values can be compared to 'null' and 'undefined'.]

with 'use strict'

And here we will execute the same code (but with 'delete g;' commented-out), this time with "use strict":

$ node strict.js
null
'a' is of type: object
'null' is of type: object
a is null, 'a' === null ? = true
undefined
'b' is of type: undefined
b is undefined, 'b' === undefined ? = true
/home/owner/Documents/node.js/Strict/strict.js:25
g = "I am a Global variable";
  ^

ReferenceError: g is not defined
    at Object.<anonymous> (/home/owner/Documents/node.js/Strict/strict.js:25:3)
    at Module._compile (internal/modules/cjs/loader.js:955:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:991:10)
    at Module.load (internal/modules/cjs/loader.js:811:32)
    at Function.Module._load (internal/modules/cjs/loader.js:723:14)
    at Function.Module.runMain (internal/modules/cjs/loader.js:1043:10)
    at internal/main/run_main_module.js:17:11
$

[As expected, "use strict" resulted in stricter syntax checking.]

Conclusion

In general, 'use strict' is very useful and can prevent lots of errors.

However, adding it to older code may not always be a good idea. It can definitely break some older code.

For instance, the this keyword can behave differently in strict mode.

Reference

Some useful references follow.

null

Read more about 'null': http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null

The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values and is treated as falsy for boolean operations.

strict mode

Read more about 'use strict': http://www.w3schools.com/js/js_strict.asp

The "use strict" directive was new in ECMAScript version 5.

ECMAScript version 5 introduced new features. To allow for backward compatibility, these new features will only apply if "use strict" is specified.

And also: http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

undefined

Read more about 'undefined': http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined

The global undefined property represents the primitive value undefined. It is one of JavaScript's primitive types.

Versions

The version of node.js used was v12.15.0:

$ node -v
v12.15.0
$

To Do

• Verify with different versions of node