tty: add NO_COLOR and FORCE_COLOR support

doc/api/tty.md

@@ -144,8 +144,9 @@ position.
 added: v9.9.0
 -->
 
-* `env` {Object} An object containing the environment variables to check.
-  **Default:** `process.env`.
+* `env` {Object} An object containing the environment variables to check. This
+  enables simulating the usage of a specific terminal. **Default:**
+  `process.env`.
 * Returns: {number}
 
 Returns:
@@ -159,11 +160,18 @@ Use this to determine what colors the terminal supports. Due to the nature of
 colors in terminals it is possible to either have false positives or false
 negatives. It depends on process information and the environment variables that
 may lie about what terminal is used.
-To enforce a specific behavior without relying on `process.env` it is possible
-to pass in an object with different settings.
+It is possible to pass in an `env` object to simulate the usage of a specific
+terminal. This can be useful to check how specific environment settings behave.
+
+To enforce a specific color support, use one of the below environment settings.
+
+* 2 colors: `FORCE_COLOR = 0` (Disables colors)
+* 16 colors: `FORCE_COLOR = 1`
+* 256 colors: `FORCE_COLOR = 2`
+* 16,777,216 colors: `FORCE_COLOR = 3`
 
-Use the `NODE_DISABLE_COLORS` environment variable to enforce this function to
-always return 1.
+Disabling color support is also possible by using the `NO_COLOR` and
+`NODE_DISABLE_COLORS` environment variables.
 
 ### writeStream.getWindowSize()
 <!-- YAML

lib/internal/tty.js

@@ -72,11 +72,49 @@ const TERM_ENVS_REG_EXP = [
   /^vt100/
 ];
 
+function warnOnDeactivatedColors(env) {
+  let name;
+  if (env.NODE_DISABLE_COLORS !== undefined)
+    name = 'NODE_DISABLE_COLORS';
+  if (env.NO_COLOR !== undefined)
+    name = 'NO_COLOR';
+
+  if (name !== undefined) {
+    process.emitWarning(
+      `The '${name}' env is ignored due to the 'FORCE_COLOR' env being set.`,
+      'Warning'
+    );
+  }
+}
+
 // The `getColorDepth` API got inspired by multiple sources such as
 // https://github.com/chalk/supports-color,
 // https://github.com/isaacs/color-support.
 function getColorDepth(env = process.env) {
-  if (env.NODE_DISABLE_COLORS || env.TERM === 'dumb') {
+  // Use level 0-3 to support the same levels as `chalk` does. This is done for
+  // consistency throughout the ecosystem.
+  if (env.FORCE_COLOR !== undefined) {
+    switch (env.FORCE_COLOR) {
+      case '':
+      case '1':
+      case 'true':
+        warnOnDeactivatedColors(env);
+        return COLORS_16;
+      case '2':
+        warnOnDeactivatedColors(env);
+        return COLORS_256;
+      case '3':
+        warnOnDeactivatedColors(env);
+        return COLORS_16m;
+      default:
+        return COLORS_2;
+    }
+  }
+
+  if (env.NODE_DISABLE_COLORS !== undefined ||
+      // See https://no-color.org/
+      env.NO_COLOR !== undefined ||
+      env.TERM === 'dumb') {
     return COLORS_2;
   }
 

test/pseudo-tty/test-tty-color-support.js

@@ -59,11 +59,18 @@ const writeStream = new WriteStream(fd);
   [{ TERM: 'color' }, 4],
   [{ TERM: 'linux' }, 4],
   [{ TERM: 'fail' }, 1],
-  [{ NODE_DISABLE_COLORS: '1' }, 1],
+  [{ TERM: 'color', NODE_DISABLE_COLORS: '1' }, 1],
   [{ TERM: 'dumb' }, 1],
   [{ TERM: 'dumb', COLORTERM: '1' }, 1],
   [{ TERM: 'terminator' }, 24],
-  [{ TERM: 'console' }, 4]
+  [{ TERM: 'console' }, 4],
+  [{ COLORTERM: '24bit', FORCE_COLOR: '' }, 4],
+  [{ NO_COLOR: '1', FORCE_COLOR: '2' }, 8],
+  [{ NODE_DISABLE_COLORS: '1', FORCE_COLOR: '3' }, 24],
+  [{ NO_COLOR: '1', COLORTERM: '24bit' }, 1],
+  [{ NO_COLOR: '', COLORTERM: '24bit' }, 1],
+  [{ TMUX: '1', FORCE_COLOR: 0 }, 1],
+  [{ NO_COLOR: 'true', FORCE_COLOR: 0, COLORTERM: 'truecolor' }, 1],
 ].forEach(([env, depth], i) => {
   const actual = writeStream.getColorDepth(env);
   assert.strictEqual(

test/pseudo-tty/test-tty-color-support.out

@@ -0,0 +1,8 @@
+(node:*) Warning: The 'NO_COLOR' env is ignored due to the 'FORCE_COLOR' env being set.
+(node:*) Warning: The 'NO_COLOR' env is ignored due to the 'FORCE_COLOR' env being set.
+(node:*) Warning: The 'NO_COLOR' env is ignored due to the 'FORCE_COLOR' env being set.
+(node:*) Warning: The 'NO_COLOR' env is ignored due to the 'FORCE_COLOR' env being set.
+(node:*) Warning: The 'NODE_DISABLE_COLORS' env is ignored due to the 'FORCE_COLOR' env being set.
+(node:*) Warning: The 'NODE_DISABLE_COLORS' env is ignored due to the 'FORCE_COLOR' env being set.
+(node:*) Warning: The 'NODE_DISABLE_COLORS' env is ignored due to the 'FORCE_COLOR' env being set.
+(node:*) Warning: The 'NODE_DISABLE_COLORS' env is ignored due to the 'FORCE_COLOR' env being set.