perf: use slice rather than substring

It is a tiny bit faster.

lib/saxes.js

@@ -575,7 +575,7 @@ class SaxesParser {
       // until we get the next chunk or the end of the stream. So save it for
       // later.
       limit--;
-      chunk = chunk.substring(0, limit);
+      chunk = chunk.slice(0, limit);
       this.trailingCR = true;
     }
 
@@ -792,7 +792,7 @@ class SaxesParser {
     }
 
     const { i } = this;
-    this[buffer] += `${chunk.substring(start, i - (this.skipTwo ? 2 : 1))}\n`;
+    this[buffer] += `${chunk.slice(start, i - (this.skipTwo ? 2 : 1))}\n`;
     return i;
   }
 
@@ -819,12 +819,12 @@ class SaxesParser {
         start = this.handleEOL(buffer, chunk, start);
       }
       else if (c === undefined) {
-        this[buffer] += chunk.substring(start);
+        this[buffer] += chunk.slice(start);
         return undefined;
       }
 
       if (chars.includes(c)) {
-        this[buffer] += chunk.substring(start, this.i - (c <= 0xFFFF ? 1 : 2));
+        this[buffer] += chunk.slice(start, this.i - (c <= 0xFFFF ? 1 : 2));
         return c;
       }
     }
@@ -851,12 +851,12 @@ class SaxesParser {
         start = this.handleEOL(buffer, chunk, start);
       }
       else if (c === undefined) {
-        this[buffer] += chunk.substring(start);
+        this[buffer] += chunk.slice(start);
         return false;
       }
 
       if (c === char) {
-        this[buffer] += chunk.substring(start, this.i - (c <= 0xFFFF ? 1 : 2));
+        this[buffer] += chunk.slice(start, this.i - (c <= 0xFFFF ? 1 : 2));
         return true;
       }
     }
@@ -876,13 +876,13 @@ class SaxesParser {
     while (true) {
       const c = this.getCode();
       if (c === undefined) {
-        this.name += chunk.substring(start);
+        this.name += chunk.slice(start);
         return undefined;
       }
 
       // NL is not a name char so we don't have to test specifically for it.
       if (!isNameChar(c)) {
-        this.name += chunk.substring(start, this.i - (c <= 0xFFFF ? 1 : 2));
+        this.name += chunk.slice(start, this.i - (c <= 0xFFFF ? 1 : 2));
         return c;
       }
     }
@@ -904,14 +904,14 @@ class SaxesParser {
     while (true) {
       const c = this.getCode();
       if (c === undefined) {
-        this[buffer] += chunk.substring(start);
+        this[buffer] += chunk.slice(start);
         return undefined;
       }
 
       // NL cannot satisfy this.nameCheck so we don't have to test
       // specifically for it.
       if (!this.nameCheck(c)) {
-        this[buffer] += chunk.substring(start, this.i - (c <= 0xFFFF ? 1 : 2));
+        this[buffer] += chunk.slice(start, this.i - (c <= 0xFFFF ? 1 : 2));
         return c;
       }
     }
@@ -1032,14 +1032,14 @@ class SaxesParser {
       switch (this.getCode()) {
       case LESS:
         this.state = S_OPEN_WAKA;
-        this.text += chunk.substring(start, this.i - 1);
+        this.text += chunk.slice(start, this.i - 1);
         forbiddenState = FORBIDDEN_START;
         // eslint-disable-next-line no-labels
         break scanLoop;
       case AMP:
         this.state = S_ENTITY;
         this.entityReturnState = S_TEXT;
-        this.text += chunk.substring(start, this.i - 1);
+        this.text += chunk.slice(start, this.i - 1);
         forbiddenState = FORBIDDEN_START;
         // eslint-disable-next-line no-labels
         break scanLoop;
@@ -1068,7 +1068,7 @@ class SaxesParser {
         forbiddenState = FORBIDDEN_START;
         break;
       case undefined:
-        this.text += chunk.substring(start);
+        this.text += chunk.slice(start);
         // eslint-disable-next-line no-labels
         break scanLoop;
       default:
@@ -1094,13 +1094,13 @@ class SaxesParser {
       switch (code) {
       case LESS:
         this.state = S_OPEN_WAKA;
-        this.text += chunk.substring(start, this.i - 1);
+        this.text += chunk.slice(start, this.i - 1);
         // eslint-disable-next-line no-labels
         break outRootLoop;
       case AMP:
         this.state = S_ENTITY;
         this.entityReturnState = S_TEXT;
-        this.text += chunk.substring(start, this.i - 1);
+        this.text += chunk.slice(start, this.i - 1);
         nonSpace = true;
         // eslint-disable-next-line no-labels
         break outRootLoop;
@@ -1109,7 +1109,7 @@ class SaxesParser {
         // eslint-disable-next-line no-labels
         break;
       case undefined:
-        this.text += chunk.substring(start);
+        this.text += chunk.slice(start);
         // eslint-disable-next-line no-labels
         break outRootLoop;
       default:
@@ -1857,15 +1857,15 @@ class SaxesParser {
     while (true) {
       const code = this.getCode();
       if (code === undefined) {
-        this.text += chunk.substring(start);
+        this.text += chunk.slice(start);
         return;
       }
 
       if (code === NL) {
         start = this.handleEOL("text", chunk, start);
       }
       else if (code === q || code === AMP || code === LESS) {
-        const slice = chunk.substring(start, this.i - 1);
+        const slice = chunk.slice(start, this.i - 1);
         switch (code) {
         case q:
           this.pushAttrib(this.name, this.text + slice);
@@ -2062,8 +2062,8 @@ class SaxesParser {
       return { prefix: "", local: name };
     }
 
-    const local = name.substring(colon + 1);
-    const prefix = name.substring(0, colon);
+    const local = name.slice(colon + 1);
+    const prefix = name.slice(0, colon);
     if (prefix === "" || local === "" || local.includes(":")) {
       this.fail(`malformed name: ${name}.`);
     }