syntax: introduce `const` support

Introduce support for declaring constant variables through the `const`
keyword. Variables declared with `const` follow the same scoping rules
as `let` declared ones.

In contrast to normal variables, `const` ones may not be assigned to
after their declaration. Any attempt to do so will result in a syntax
error during compilation.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 15 insertions, 1 deletions

diff --git a/README.md b/README.md
index 55ac9c3..fc3c1da 100644
--- a/README.md
+++ b/README.md
@@ -154,9 +154,14 @@ through numeric operations, or explicitely, e.g. by invoking functions such as
 
 Variable names must start with a letter or an underscore and may only contain
 the characters `A`..`Z`, `a`..`z`, `0`..`9` or `_`. By prefixing a variable
-name with the keyword `let`, it is declared in the local function scope only
+name with the keyword `let`, it is declared in the local block scope only
 and not visible outside anymore.
 
+Variables may also be declared using the `const` keyword. Such variables follow
+the same scoping rules as `let` declared ones but they cannot be modified after
+they have been declared. Any attempt to do so will result in a syntax error
+during compilation.
+
 ```javascript
 {%
 
@@ -172,6 +177,15 @@ and not visible outside anymore.
   print(a, "\n");  // outputs "2"
   print(b, "\n");  // outputs nothing
 
+  const c = 3;
+  print(c, "\n");  // outputs "3"
+
+  c = 4;           // raises syntax error
+  c++;             // raises syntax error
+
+  const d;         // raises syntax error, const variables must
+                   // be initialized at declaration time
+
 %}
 ```