WIP

src/sass/lib/css-vars.scss

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+// Downloaded from https://github.com/malyw/css-vars
+
+//// VARIABLES ////
+
+// global map to be filled via variables
+$css-vars: ();
+
+// the variable may be set to "true" anywhere in the code,
+// so native CSS custom properties will be used instead of the Sass global map
+$css-vars-use-native: false !default;
+
+// enables the output of debug messages
+$css-vars-debug-log: false !default;
+
+//// FUNCTIONS ////
+
+///
+// Assigns a variable to the global map
+///
+@function _cssVarAssign($varName: null, $varValue: null) {
+    // CHECK PARAMS
+    @if ($varName==null) {
+        @error 'Variable name is expected, instead got: null';
+    }
+    @if ($varValue==null) {
+        @error 'Variable value is expected, instead got: null';
+    }
+
+    // assign to the global map
+    @if ($css-vars-debug-log and map-get($css-vars, $varName)) {
+        @debug "'#{$varName}' variable is reassigned";
+    }
+
+    @return map-merge($css-vars, ($varName: $varValue));
+}
+
+///
+// Emulates var() CSS native function behavior
+//
+// $args[0] {String} "--" + variable name
+// [$args[1]] Optional default value if variable is not assigned yet
+//
+// E.G.:
+// color: var(--main-color);
+// background: var(--main-bg, green);
+///
+@function var($args...) {
+    // CHECK PARAMS
+    @if (length($args) ==0) {
+        @error 'Variable name is expected to be passed to the var() function';
+    }
+    @if (str-length(nth($args, 1)) < 2 or str-slice(nth($args, 1), 0, 2) != '--') {
+        @error "Variable name is expected to start from '--'";
+    }
+
+    // PROCESS
+    $varName: nth($args, 1);
+    $varValue: map-get($css-vars, $varName);
+
+    @if ($css-vars-debug-log or not $css-vars-use-native) {
+        // Sass or debug
+        @if ($varValue==null) {
+            // variable is not provided so far
+            @if (length($args) ==2) {
+                // the default value is passed
+                @if ($css-vars-debug-log) {
+                    @debug "Provided default value is used for the variable: '#{$varName}'";
+                }
+                $varValue: nth($args, 2);
+            } @else if ($css-vars-debug-log) {
+                @debug "Variable '#{$varName}' is not assigned";
+                @if (not $css-vars-use-native) {
+                    @debug "The 'var(#{$varName}...)' usage will be skipped in the output CSS";
+                }
+            }
+        }
+    }
+
+    @if ($css-vars-use-native) {
+        // CSS variables
+        // Native CSS: don't process function in case of native
+        @return unquote('var(' + $args + ')');
+    } @else {
+        // Sass: return value from the map
+        @return $varValue;
+    }
+}
+
+//// MIXIN ////
+
+///
+// CSS mixin to provide variables
+// E.G.:
+// @include css-vars((
+//    --color: rebeccapurple,
+//    --height: 68px,
+//    --margin-top: calc(2vh + 20px)
+// ));
+///
+@mixin css-vars($varMap: null) {
+    // CHECK PARAMS
+    @if ($varMap==null) {
+        @error 'Map of variables is expected, instead got: null';
+    }
+    @if (type_of($varMap) !=map) {
+        @error 'Map of variables is expected, instead got another type passed: #{type_of($varMap)}';
+    }
+
+    // PROCESS
+    @if ($css-vars-debug-log or not $css-vars-use-native) {
+        // Sass or debug
+        // merge variables and values to the global map (provides no output)
+        @each $varName, $varValue in $varMap {
+            $css-vars: _cssVarAssign($varName, $varValue) !global; // store in global variable
+        }
+    }
+
+    @if ($css-vars-use-native) {
+        // CSS variables
+        // Native CSS: assign CSS custom properties to the global scope
+        @at-root :root {
+            @each $varName, $varValue in $varMap {
+                @if (type_of($varValue) ==string) {
+                    #{$varName}: $varValue; // to prevent quotes interpolation
+                } @else {
+                    #{$varName}: #{$varValue};
+                }
+            }
+        }
+    }
+}